Avoid directly accessing SPIR-V instruction words. The helper methods
provide self-explanatory semantics so we can eliminate local variables
to store these IDs.

Bug: b/129000021
Change-Id: Ie42782d53b9c24014b6a1b1f51b82085b6c2ebef
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43694
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>