For compile time indices, we can do some basic sanity checking.

Change-Id: I9ae4ea5bbf106c4653cbf9bbe6776a5a188b4289
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33470Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>