Allow Offset in addition to ConstOffset for texture operations

[rg3igalia]

As part of an upcoming Vulkan maintenance extension I'm developing CTS tests for, we want to allow using Offset in addition to ConstOffset for image operations. This was already possible for image gather operations, but not for others, and is mainly enforced by VUID-StandaloneSpirv-Offset-04663. There are other VUIDs affected by this but that's the main one. On the SPIR-V side there's no such restriction, so SPIR-V doesn't need any special capability nor a new extension to support this behavior.

While developing CTS tests I managed to get what I wanted from glslang by applying the following diff.

diff --git a/glslang/MachineIndependent/ParseHelper.cpp b/glslang/MachineIndependent/ParseHelper.cpp
index 215ccd99..b187ce8a 100644
--- a/glslang/MachineIndependent/ParseHelper.cpp
+++ b/glslang/MachineIndependent/ParseHelper.cpp
@@ -2332,9 +2332,7 @@ void TParseContext::builtInOpCheck(const TSourceLoc& loc, const TFunction& fnCan
                                     arg0->getType().getSampler().shadow;
             if (f16ShadowCompare)
                 ++arg;
-            if (! (*argp)[arg]->getAsTyped()->getQualifier().isConstant())
-                error(loc, "argument must be compile-time constant", "texel offset", "");
-            else if ((*argp)[arg]->getAsConstantUnion()) {
+            else if (((*argp)[arg]->getAsTyped()->getQualifier().isConstant()) && (*argp)[arg]->getAsConstantUnion()) {
                 const TType& type = (*argp)[arg]->getAsTyped()->getType();
                 for (int c = 0; c < type.getVectorSize(); ++c) {
                     int offset = (*argp)[arg]->getAsConstantUnion()->getConstArray()[c].getIConst();

But I'm not used to glslang's code so I don't know if my changes are correct and complete, and I'm fairly positive I'm breaking several tests with that change. Could someone take a look at this? Ideally I'd like glslang to support this so I can just update the glslang commit reference in CTS and know this is going to work.

/cc @spencer-lunarg since this is related to validation too.