[InstCombine] Add simplification of two logical and/ors

This is a patch that adds folding of two logical and/ors that share one variable:

a && (a && b) -> a && b
a && (a & b)  -> a && b
...

This is towards removing the poison-unsafe select optimization (D93065 has more context).

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D96945