Use these for a fast-path for Load() and Store().

This is an attempt to fix the severe performance hit we incurred with robustness.

Bug: b/131224163
Change-Id: I3e244bed5ed723cf29538ff022781c813caaa5eb
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29454Tested-by: Ben Clayton <bclayton@google.com>
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>