arch-gcn3: Explicitly sign-extend simm16

arch-gcn3: Explicitly sign-extend simm16

In some instructions, simm16 needs to be sign extended. Previous code
simply casted the simm16 to a 32-bit or 64-bit datatype, however this
didn't actually sign-extend the value.

This patch explicitly calls sext<16> on simm16 whenever it's supposed
to be sign-extended.

Change-Id: I32f02e51fbab220d1a73dc7e68c7410937db21c7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37495
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>