[amdgpu] Change the RA to basic

Specifying `-regalloc=fast` is not reliable. With fast register allocation,
`LIS = getAnalysisIfAvailable<LiveIntervals>();` get nullptr
in "si-lower-sgpr-spills" pass, so the slot index is not created in the
pass for new inserted instructions. When verifying the machine
instructions, it fails on checking slot index. While greedy-ra is time
consuming basic-ra can be used to reduce compiling time for this test case.

Differential Revision: https://reviews.llvm.org/D131931