Recommit "[ConstraintElim] Treat ConstantPointerNull as constant offset 0."

This reverts commit 58286f9.

Now with the test correctly updated.

Original message:
    Treat ConstantPointerNull (null) as constant offset 0 in the constraint
    instead of a variable. This slightly reduces the number of variables. I
    was not able to find a test case where this actually caused any changes.

Author: fhahn

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

@@ -455,6 +455,9 @@ static Decomposition decompose(Value *V,
   if (Ty->isPointerTy() && !IsSigned) {
     if (auto *GEP = dyn_cast<GEPOperator>(V))
       return decomposeGEP(*GEP, Preconditions, IsSigned, DL);
+    if (isa<ConstantPointerNull>(V))
+      return int64_t(0);
+
     return V;
   }
 

llvm/test/Transforms/ConstraintElimination/reproducer-remarks-debug.ll

@@ -5,7 +5,7 @@
 target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 
 ; CHECK:      Condition   %c.2 = icmp eq ptr %a, null implied by dominating constraints
-; CHECK-NEXT: %a + -1 * null <= 0
+; CHECK-NEXT: %a <= 0
 ; CHECK-NEXT: Creating reproducer for   %c.2 = icmp eq ptr %a, null
 ; CHECK-NEXT:   found external input ptr %a
 ; CHECK-NEXT:   Materializing assumption icmp eq ptr %a, null