[AMDGPU] Add test coverage to ensure first regallocfast only allocates SGPR

Register allocation is split into two passes, and the expected behavior is that the first pass only should only work on virtual SGPRs. Whereas the second pass works on virtual VGPRs. This adds a test case which breaks if the first pass allocates VGPRs.

Differential Revision: https://reviews.llvm.org/D135331