[BasicAA] Optimize index size adjustment (NFC)

In most cases we do not actually have to perform an index size
adjustment. Don't perform any APInt operations in that case.

Author: nikic

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -457,10 +457,13 @@ static LinearExpression GetLinearExpression(
 /// an issue, for example, in particular for 32b pointers with negative indices
 /// that rely on two's complement wrap-arounds for precise alias information
 /// where the maximum index size is 64b.
-static APInt adjustToIndexSize(const APInt &Offset, unsigned IndexSize) {
+static void adjustToIndexSize(APInt &Offset, unsigned IndexSize) {
   assert(IndexSize <= Offset.getBitWidth() && "Invalid IndexSize!");
   unsigned ShiftBits = Offset.getBitWidth() - IndexSize;
-  return (Offset << ShiftBits).ashr(ShiftBits);
+  if (ShiftBits != 0) {
+    Offset <<= ShiftBits;
+    Offset.ashrInPlace(ShiftBits);
+  }
 }
 
 namespace {
@@ -685,7 +688,7 @@ BasicAAResult::DecomposeGEPExpression(const Value *V, const DataLayout &DL,
 
       // Make sure that we have a scale that makes sense for this target's
       // index size.
-      Scale = adjustToIndexSize(Scale, IndexSize);
+      adjustToIndexSize(Scale, IndexSize);
 
       if (!!Scale) {
         VariableGEPIndex Entry = {LE.Val, Scale, CxtI, LE.IsNSW,
@@ -696,7 +699,7 @@ BasicAAResult::DecomposeGEPExpression(const Value *V, const DataLayout &DL,
 
     // Take care of wrap-arounds
     if (GepHasConstantOffset)
-      Decomposed.Offset = adjustToIndexSize(Decomposed.Offset, IndexSize);
+      adjustToIndexSize(Decomposed.Offset, IndexSize);
 
     // Analyze the base pointer next.
     V = GEPOp->getOperand(0);