[CVP] Simplify comparisons without constant operand

CVP currently only tries to simplify comparisons if there is a
constant operand. However, even if both are non-constant, we may
be able to determine the result of the comparison based on range
information.

IPSCCP is already capable of doing this, but because it runs very
early, it may miss some cases.

Differential Revision: https://reviews.llvm.org/D137253