[AMDGPU] Add IntrWillReturn to three intrinsics

None of these can terminate a wave or lane.
With these, all intrinsic are IntrWillReturn except those that change
exec or can terminate the wave.

Not marking intrinsics as WillReturn may prevent optimizations in the
future: https://lists.llvm.org/pipermail/llvm-dev/2021-January/148047.html

Differential Revision: https://reviews.llvm.org/D95436