[InstCombine] Fold memcmp of constant arrays and variable size

The memcmp simplifier is limited to folding to constants calls with constant
arrays and constant sizes.  This change adds the ability to simplify
memcmp(A, B, N) calls with constant A and B and variable N to the pseudocode
equivalent of

N <= Pos ? 0 : (A < B ? -1 : B < A ? +1 : 0)

where Pos is the offset of the first mismatch between A and B.

Differential Revision: https://reviews.llvm.org/D127766