[InstCombine] Avoid use of ConstantExpr::getZExtOrBitcast() (NFC)

Use the constant folding API instead. In the second case using
IR builder should also work, but the way the instructions are
created an inserted there is very unusual, so I've left it alone.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -4246,7 +4246,12 @@ foldShiftIntoShiftInAnotherHandOfAndInICmp(ICmpInst &I, const SimplifyQuery SQ,
                       /*isNUW=*/false, SQ.getWithInstruction(&I)));
   if (!NewShAmt)
     return nullptr;
-  NewShAmt = ConstantExpr::getZExtOrBitCast(NewShAmt, WidestTy);
+  if (NewShAmt->getType() != WidestTy) {
+    NewShAmt =
+        ConstantFoldCastOperand(Instruction::ZExt, NewShAmt, WidestTy, SQ.DL);
+    if (!NewShAmt)
+      return nullptr;
+  }
   unsigned WidestBitWidth = WidestTy->getScalarSizeInBits();
 
   // Is the new shift amount smaller than the bit width?

llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -136,9 +136,14 @@ Value *InstCombinerImpl::reassociateShiftAmtsOfTwoSameDirectionShifts(
 
   assert(IdenticalShOpcodes && "Should not get here with different shifts.");
 
-  // All good, we can do this fold.
-  NewShAmt = ConstantExpr::getZExtOrBitCast(NewShAmt, X->getType());
+  if (NewShAmt->getType() != X->getType()) {
+    NewShAmt = ConstantFoldCastOperand(Instruction::ZExt, NewShAmt,
+                                       X->getType(), SQ.DL);
+    if (!NewShAmt)
+      return nullptr;
+  }
 
+  // All good, we can do this fold.
   BinaryOperator *NewShift = BinaryOperator::Create(ShiftOpcode, X, NewShAmt);
 
   // The flags can only be propagated if there wasn't a trunc.