This wasn't considering that the offsets are bytes and not whole words.
Another bug that wasn't caught by dEQP (this would only affect atomic load / stores).

Bug: b/131224163
Change-Id: I6356da3ecb86cdbb7a25667fe617f650019950e2
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33168Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>