[InstCombine] Fold memchr and strchr equality with first argument

Enhance memchr and strchr handling to simplify calls to the functions
used in equality expressions with the first argument to at most two
integer comparisons:

- memchr(A, C, N) == A to N && *A == C for either a dereferenceable
  A or a nonzero N,
- strchr(S, C) == S to *S == C for any S and C, and
- strchr(S, '\0') == 0 to true for any S

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D128939