Clarify DepthGreater/Less/Unchanged execution modes are unused
Clarify DepthGreater/Less/Unchanged execution modes are unused

These SPIR-V execution modes can be used to optimize early depth test,
but are currently ignored, which is fine from a correctness point of
view.

We need to handle these execution modes because they can appear in any
shader, without the implementation having to support using them for
optimizations. Also, we can keep the SpirvShader::Modes fields that
correspond to them for debugging purposes (e.g. an application might
incorrectly rely on early depth test optimization to happen).

Bug: b/177915067
Change-Id: I851fdfbcdde5be6287bc4ee689fdbd6549e2900f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51908
Presubmit-Ready: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Tested-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Kokoro-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>

bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader