arch-gcn3: Fix operand size reporting for Flat insts

arch-gcn3: Fix operand size reporting for Flat insts

Some Flat instructions were reporting their operand sizes in bits
instead of bytes. This lead to panics occuring in
StaticRegisterManagerPolicy::mapVgpr.

This patch updates those insts to report their operand sizes in bytes.

Change-Id: I48f485e638864a1f2a1a3be66ed20893e73e9705
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36275
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Alexandru Duțu <alexandru.dutu@amd.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>