[SCEV] Split collecting and applying rewrite info from loop guards (NFC) (#97316)

Introduce a new LoopGuards class to track info from loop guards and split 
off collecting rewrite info to LoopGuards::collect. This allows users of 
applyLoopGuards to collect rewrite info once in cases where the same
loop guards are applied multiple times.

This is used to collect rewrite info once in howFarToZero, which saves a
bit of compile-time:
stage1-O3: -0.04%
stage1-ReleaseThinLTO: -0.02%
stage1-ReleaseLTO-g: -0.04%
stage2-O3: -0.02%

https://llvm-compile-time-tracker.com/compare.php?from=117b53ae38428ca66eaa886fb432e6f09db88fe4&to=4ffb7b2e1c99081ccebe6f236c48a0be2f64b6ff&stat=instructions:u

Notably this improves mafft by -0.9% with -O3, -0.11% with LTO and
-0.12% with stage2-O3.

PR: #97316

Author: fhahn

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -1299,8 +1299,26 @@ class ScalarEvolution {
   /// sharpen it.
   void setNoWrapFlags(SCEVAddRecExpr *AddRec, SCEV::NoWrapFlags Flags);
 
+  class LoopGuards {
+    DenseMap<const SCEV *, const SCEV *> RewriteMap;
+    bool PreserveNUW = false;
+    bool PreserveNSW = false;
+    ScalarEvolution &SE;
+
+    LoopGuards(ScalarEvolution &SE) : SE(SE) {}
+
+  public:
+    /// Collect rewrite map for loop guards for loop \p L, together with flags
+    /// indicating if NUW and NSW can be preserved during rewriting.
+    static LoopGuards collect(const Loop *L, ScalarEvolution &SE);
+
+    /// Try to apply the collected loop guards to \p Expr.
+    const SCEV *rewrite(const SCEV *Expr) const;
+  };
+
   /// Try to apply information from loop guards for \p L to \p Expr.
   const SCEV *applyLoopGuards(const SCEV *Expr, const Loop *L);
+  const SCEV *applyLoopGuards(const SCEV *Expr, const LoopGuards &Guards);
 
   /// Return true if the loop has no abnormal exits. That is, if the loop
   /// is not infinite, it must exit through an explicit edge in the CFG.