[InstCombine] Fold strncmp of constant arrays and variable size

Extend the solution accepted in D127766 to strncmp and simplify
strncmp(A, B, N) calls with constant A and B and variable N to
the equivalent of

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

where Pos is the offset of either the first mismatch between A
and B or the terminating null character if both A and B are equal
strings.

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D128089