[IR] Don't create ptrtoint expression to determine alignment (NFCI) (#161364)

We try to determine the alignment of a constant by creating a ptrtoint
expression and seeing if it folds. I believe the only case this can
actually handle is where the constant is an inttoptr expression. Handle
that directly instead of going through another ptrtoint expression.

I ran into this while trying to clean up our isEliminableCastPair()
mess, which is going to disable ptrtoint(inttoptr) folding without
DataLayout, breaking this code.

Author: nikic

llvm/lib/IR/Value.cpp

@@ -1000,14 +1000,12 @@ Align Value::getPointerAlignment(const DataLayout &DL) const {
       ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(0));
       return Align(CI->getLimitedValue());
     }
-  } else if (auto *CstPtr = dyn_cast<Constant>(this)) {
-    // Strip pointer casts to avoid creating unnecessary ptrtoint expression
-    // if the only "reduction" is combining a bitcast + ptrtoint.
-    CstPtr = CstPtr->stripPointerCasts();
-    if (auto *CstInt = dyn_cast_or_null<ConstantInt>(ConstantExpr::getPtrToInt(
-            const_cast<Constant *>(CstPtr), DL.getIntPtrType(getType()),
-            /*OnlyIfReduced=*/true))) {
-      size_t TrailingZeros = CstInt->getValue().countr_zero();
+  } else if (auto *CE = dyn_cast<ConstantExpr>(this)) {
+    // Determine the alignment of inttoptr(C).
+    if (CE->getOpcode() == Instruction::IntToPtr &&
+        isa<ConstantInt>(CE->getOperand(0))) {
+      ConstantInt *IntPtr = cast<ConstantInt>(CE->getOperand(0));
+      size_t TrailingZeros = IntPtr->getValue().countr_zero();
       // While the actual alignment may be large, elsewhere we have
       // an arbitrary upper alignmet limit, so let's clamp to it.
       return Align(TrailingZeros < Value::MaxAlignmentExponent