Scalar: Ignore ConstantData in processAssumption

Assumptions on UndefValue and ConstantPointerNull aren't relevant to
other users.  Ignore them entirely to avoid wasting cycles walking
through their (possibly extremely extensive (cross-module)) use-lists.

It wasn't clear how to add a specific test for this, and it'll be
covered anyway by an eventual patch that asserts when trying to access
the use-list of an instance of ConstantData.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282334 91177308-0d34-0410-b5e6-96231b3b80d8

Author: dexonsmith

lib/Transforms/Scalar/AlignmentFromAssumptions.cpp

@@ -297,6 +297,11 @@ bool AlignmentFromAssumptionsPass::processAssumption(CallInst *ACall) {
   if (!extractAlignmentInfo(ACall, AAPtr, AlignSCEV, OffSCEV))
     return false;
 
+  // Skip ConstantPointerNull and UndefValue.  Assumptions on these shouldn't
+  // affect other users.
+  if (isa<ConstantData>(AAPtr))
+    return false;
+
   const SCEV *AASCEV = SE->getSCEV(AAPtr);
 
   // Apply the assumption to all other users of the specified pointer.