[InstCombine] substitute equivalent constant to reduce logic-of-icmps

(X == C) && (Y Pred1 X) --> (X == C) && (Y Pred1 C)
(X != C) || (Y Pred1 X) --> (X != C) || (Y Pred1 C)

This cooperates/overlaps with D78430, but it is a more general transform
that gets us most of the expected simplifications and several other
improvements.
http://volta.cs.utah.edu:8080/z/5gxjjc

PR45618:
https://bugs.llvm.org/show_bug.cgi?id=45618

Differential Revision: https://reviews.llvm.org/D78582