[AMDGPU][GISel] Re-enable some working tests

These tests had been commented out but seem to not be crashing.
Not sure if codegen is perfect in each of them, but even if it's not I think it's better to put a TODO to fix codegen than remove the test outright, unless codegen is plain wrong (then I'd still rather XFAIL rather than hide it)

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D136341