arch-vega: Fix SOPK instruction sign extends

See: https://gem5-review.googlesource.com/c/public/gem5/+/37495

Same patch but for vega. This fixes issues with lulesh and probably
rodinia - heartwall as well in fullsystem.

Change-Id: I3af36bb9b60d32dc96cc3b439bb1167be1b0945d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/65432
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
(cherry picked from commit 8693d725e202002893aafc4ac814bfa87c86ae76)
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/65453
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>