[NFC][AMDGPU] Add tests for all the various IR patterns equivalent to extracting low bits.

Summary:
The idiom recognition seems rather poor.
Only the `@bzhi32_d0` produces `v_bfe_u32`.
But they all should.

This needs to be fixed before D47980 can be re-landed.

Reviewers: mareko, bogner, rampitec, arsenm, tstellar, nhaehnle

Reviewed By: nhaehnle

Subscribers: kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Tags: #amdgpu

Differential Revision: https://reviews.llvm.org/D48005

llvm-svn: 334398