The SPIR-V spec states that the Sample operand of OpImageTexelPointer
"must be a valid <id> for the value 0 if the OpTypeImage has MS of 0."

This change optimizes for this common case by no performing texel
address sample offset and bounds check calculations.

Bug: b/163142358
Change-Id: I75932c264d45df1012fa8451fc4c57671191b59b
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/47808
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>