arch-gcn3: Free registers when execMask = 0

arch-gcn3: Free registers when execMask = 0

Flat instructions free some of their registers through their memory
requests, in particuar a call to scheduleWriteOperandsFromLoad(),
which gets called from GlobalMemPipeline::exec.

When execMask is 0, the instruction doesn't issue a memory request.

This patch adds in a call to scheduleWriteOperandsFromLoad() when
execMask is 0 for Flat Load and AtomicReturn instructions, as those
are the instructions that call scheduleWriteOperandsFromLoad()
in the memory pipeline.

This patch also adds in a missing return statement when execMask is 0
in one of the Flat instructions.

Change-Id: I09296adb7401e7515d3cedceb780a5df4598b109
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32234
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>