[llvm] [AMDGPU] Modify Dyn Alloca test to account for Machine-Verifier behavior (PR #120393)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> his is due to the buffer resource descriptor selected
during G-ISEL, before the fallback path.
Device functions use $sgpr0_sgpr1_sgpr2_sgpr3.
while Kernel functions select $private_rsrc_reg
where machine-verifier complains:
$private_rsrc_reg is not a SReg_128 register.


This is just a bug that should be fixed, not a behavior to consider 

https://github.com/llvm/llvm-project/pull/120393