Also separate the out-of-bounds handling from the offset calculations.

Bug: b/155862459
Change-Id: Ib6c5ea029d16fc411d792e6c129b37386d48fc1d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/45092Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>