AMDGPU: Fix a slow test by using basic regalloc
AMDGPU: Fix a slow test by using basic regalloc

This just tests that the register limit isn't exceeded,
so the regisetr allocation doesn't need to be great.'

The critically slow part is all in greedy RA, so
switch to basic.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@277700 91177308-0d34-0410-b5e6-96231b3b80d8

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