gpu-compute: set exec_mask for permute,bpermute instructions

gpu-compute: set exec_mask for permute,bpermute instructions

This change sets gpuDynInst->exec_mask for permute and bpermute
instructions, fixing a bug where they would never write their data.

permute and bpermute instructions are load instructions that write
to a VGPR. Because of that, they use gpuDynInst->exec_mask when
checking what lanes should write to the VGPR.

gpuDynInst->exec_mask gets set to wf->execMask() as that is what other
load instructions that write to VGPRs do.

Change-Id: Ie443283488cbd2ab9c17fc255e7cc44418353419
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35036
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>