SPV constants and constOffsets completion: isConstant() and disassembler to see results.

Expand to full isConstant() implementation.
Fix disassembler to generate texture look-up masks.

SPIRV/disassemble.cpp

@@ -398,7 +398,7 @@ void SpirvStream::disassembleInstruction(Id resultId, Id /*typeId*/, Op opCode,
             disassembleIds(numOperands);
             return;
         case OperandOptionalImage:
-            outputMask(operandClass, stream[word++]);
+            outputMask(OperandImageOperands, stream[word++]);
             --numOperands;
             disassembleIds(numOperands);
             return;