[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

llvm-svn: 305734