[SCEV] Introduce SCEVUse, use it instead of const SCEV * (NFCI) (WIP).

This patch introduces SCEVUse, which is a tagged pointer containing the
used const SCEV *, plus extra bits to store NUW/NSW flags that are only
valid at the specific use.

This was suggested by @nikic as an alternative
to #90742.

This patch just updates most SCEV infrastructure to operate on SCEVUse
instead of const SCEV *. It does not introduce any code that makes use
of the use-specific flags yet which I'll share as follow-ups.

Note that this should be NFC, but currently there's at least one case
where it is not (turn-to-invariant.ll), which I'll investigate once we
agree on the overall direction.

This PR at the moment also contains a commit that updates various SCEV
clients to use `const SCEV *` instead of `const auto *`, to prepare for
this patch. This reduces the number of changes needed, as SCEVUse will
automatically convert to `const SCEV *`. This is a safe default, as it
just drops the use-specific flags for the expression (it will not drop
any use-specific flags for any of its operands though).

This probably
SCEVUse could probably also be used to address mis-compiles due to
equivalent AddRecs modulo flags result in an AddRec with incorrect flags
for some uses of some phis, e.g. the one
#80430 attempted to fix

Compile-time impact:
stage1-O3: +0.06%
stage1-ReleaseThinLTO: +0.07%
stage1-ReleaseLTO-g: +0.07%
stage2-O3: +0.11%

https://llvm-compile-time-tracker.com/compare.php?from=ce055843e2be9643bd58764783a7bb69f6db8c9a&to=8c7f4e9e154ebc4862c4e2716cedc3c688352d7c&stat=instructions:u

Author: fhahn

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -1250,10 +1250,12 @@ bool DependenceInfo::strongSIVtest(const SCEV *Coeff, const SCEV *SrcConst,
   if (const SCEV *UpperBound = collectUpperBound(CurLoop, Delta->getType())) {
     LLVM_DEBUG(dbgs() << "\t    UpperBound = " << *UpperBound);
     LLVM_DEBUG(dbgs() << ", " << *UpperBound->getType() << "\n");
-    const SCEV *AbsDelta =
-      SE->isKnownNonNegative(Delta) ? Delta : SE->getNegativeSCEV(Delta);
-    const SCEV *AbsCoeff =
-      SE->isKnownNonNegative(Coeff) ? Coeff : SE->getNegativeSCEV(Coeff);
+    const SCEV *AbsDelta = SE->isKnownNonNegative(Delta)
+                               ? Delta
+                               : SE->getNegativeSCEV(Delta).getPointer();
+    const SCEV *AbsCoeff = SE->isKnownNonNegative(Coeff)
+                               ? Coeff
+                               : SE->getNegativeSCEV(Coeff).getPointer();
     const SCEV *Product = SE->getMulExpr(UpperBound, AbsCoeff);
     if (isKnownPredicate(CmpInst::ICMP_SGT, AbsDelta, Product)) {
       // Distance greater than trip count - no dependence
@@ -1791,8 +1793,9 @@ bool DependenceInfo::weakZeroSrcSIVtest(const SCEV *DstCoeff,
   const SCEV *AbsCoeff =
     SE->isKnownNegative(ConstCoeff) ?
     SE->getNegativeSCEV(ConstCoeff) : ConstCoeff;
-  const SCEV *NewDelta =
-    SE->isKnownNegative(ConstCoeff) ? SE->getNegativeSCEV(Delta) : Delta;
+  const SCEV *NewDelta = SE->isKnownNegative(ConstCoeff)
+                             ? SE->getNegativeSCEV(Delta).getPointer()
+                             : Delta;
 
   // check that Delta/SrcCoeff < iteration count
   // really check NewDelta < count*AbsCoeff
@@ -1900,8 +1903,9 @@ bool DependenceInfo::weakZeroDstSIVtest(const SCEV *SrcCoeff,
   const SCEV *AbsCoeff =
     SE->isKnownNegative(ConstCoeff) ?
     SE->getNegativeSCEV(ConstCoeff) : ConstCoeff;
-  const SCEV *NewDelta =
-    SE->isKnownNegative(ConstCoeff) ? SE->getNegativeSCEV(Delta) : Delta;
+  const SCEV *NewDelta = SE->isKnownNegative(ConstCoeff)
+                             ? SE->getNegativeSCEV(Delta).getPointer()
+                             : Delta;
 
   // check that Delta/SrcCoeff < iteration count
   // really check NewDelta < count*AbsCoeff

llvm/include/llvm/Analysis/ScalarEvolutionExpressions.h

@@ -1485,7 +1485,7 @@ bool InductionDescriptor::isInductionPHI(
     return false;
 
   // Check that the PHI is consecutive.
-  const SCEV *PhiScev = Expr ? Expr : SE->getSCEV(Phi);
+  const SCEV *PhiScev = Expr ? Expr : SE->getSCEV(Phi).getPointer();
   const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(PhiScev);
 
   if (!AR) {

llvm/lib/Analysis/DependenceAnalysis.cpp

@@ -494,7 +494,8 @@ bool IndexedReference::isConsecutive(const Loop &L, const SCEV *&Stride,
                          SE.getNoopOrSignExtend(ElemSize, WiderType));
   const SCEV *CacheLineSize = SE.getConstant(Stride->getType(), CLS);
 
-  Stride = SE.isKnownNegative(Stride) ? SE.getNegativeSCEV(Stride) : Stride;
+  Stride = SE.isKnownNegative(Stride) ? SE.getNegativeSCEV(Stride).getPointer()
+                                      : Stride;
   return SE.isKnownPredicate(ICmpInst::ICMP_ULT, Stride, CacheLineSize);
 }