[AMDGPU] Add TII::isIgnorableUse() to allow VOP rematerialization

[AMDGPU] Add TII::isIgnorableUse() to allow VOP rematerialization

Any def of EXEC prevents rematerialization of any VOP instruction
because of the physreg use. Create a callback to check if the
physreg use can be ingored to allow rematerialization.

Differential Revision: https://reviews.llvm.org/D105836