[AMDGPU] Allow hoisting of some VALU compare instructions

Conversatively allow hoisting/sinking of VALU comparisons.
If the result of a comparison is masked with exec, narrowing the
set of active lanes, then it is safe to hoist it as the masking
instruction will never by hoisted.

Heuristically this is also true for sinking, as we do not expect
the result of a sunk comparison that is masked with exec to be
used outside of the loop.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D118975