[LibCallSimplifier] try harder to fold memcmp with constant arguments

Try to fold:
memcmp(X, C, ConstantLength) == 0 --> load X == *C

Without this change, we're unnecessarily checking the alignment of the constant data, 
so we miss the transform in the first 2 tests in the patch.

I noted this shortcoming of LibCallSimpifier in one of the recent CGP memcmp expansion 
patches. This doesn't help the example in:
https://bugs.llvm.org/show_bug.cgi?id=34032#c13
...directly, but it's worth short-circuiting more of these simple cases since we're 
already trying to do that.

The benefit of transforming to load+cmp is that existing IR analysis/transforms may
further simplify that code. For example, if the load of the variable is common to 
multiple memcmp calls, CSE can remove the duplicate instructions.

Differential Revision: https://reviews.llvm.org/D36922

llvm-svn: 311333