[TTI][AArch64] Detect OperandInfo from scalable splats.

Pulled out of llvm#122236, this allows Splats contants to be recognized in by
getOperandInfo, allowing "better" costs for instructions like divides by
constants to be produced (which are expanded into mul+add+shift). Some of the
costs are not very accurate yet, but the comparison of scalar vs fixed-with vs
scalable for the same fiv can become more accurate, especially with patches
like llvm#122236.

Author: davemgreen

llvm/lib/Analysis/TargetTransformInfo.cpp

@@ -893,7 +893,8 @@ TargetTransformInfo::getOperandInfo(const Value *V) {
 
   // Check for a splat of a constant or for a non uniform vector of constants
   // and check if the constant(s) are all powers of two.
-  if (isa<ConstantVector>(V) || isa<ConstantDataVector>(V)) {
+  if (isa<ConstantVector>(V) || isa<ConstantDataVector>(V) ||
+      isa<ConstantExpr>(V)) {
     OpInfo = OK_NonUniformConstantValue;
     if (Splat) {
       OpInfo = OK_UniformConstantValue;