[CGP, PowerPC] try to constant fold before creating loads for memcmp expansion

[CGP, PowerPC] try to constant fold before creating loads for memcmp expansion

This is the last step needed to avoid regressions for x86 before we flip the switch to allow
expansion of the smallest set of memcpy() via CGP. The DAG version checks for constant strings,
so we need to do that here too.

FWIW, the 2 constant test is not handled by LibCallSimplifier::optimizeMemCmp() because that
code is limited to 8-bit constant arrays. LibCallSimplifier will also fail to optimize some 1
constant tests because its alignment requirements are too strict (shouldn't require alignment
for a constant operand).

Differential Revision: https://reviews.llvm.org/D34071

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305734 91177308-0d34-0410-b5e6-96231b3b80d8