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/SpvBuilder.cpp

@@ -500,6 +500,27 @@ Id Builder::findScalarConstant(Op typeClass, Id typeId, unsigned v1, unsigned v2
     return 0;
 }
 
+bool Builder::isConstantOpCode(Op opcode) const
+{
+    switch (opcode) {
+    case OpUndef: 
+    case OpConstantTrue:
+    case OpConstantFalse:
+    case OpConstant:
+    case OpConstantComposite:
+    case OpConstantSampler:
+    case OpConstantNull:
+    case OpSpecConstantTrue:
+    case OpSpecConstantFalse:
+    case OpSpecConstant:
+    case OpSpecConstantComposite:
+    case OpSpecConstantOp:
+        return true;
+    default:
+        return false;
+    }
+}
+
 Id Builder::makeBoolConstant(bool b)
 {
     Id typeId = makeBoolType();
@@ -1194,7 +1215,6 @@ Id Builder::createTextureCall(Decoration precision, Id resultType, bool fetch, b
             mask = (ImageOperandsMask)(mask | ImageOperandsOffsetMask);
         texArgs[numArgs++] = parameters.offset;
     }
-    // TBD: if Offset is constant, use ImageOperandsConstOffsetMask
     if (parameters.offsets) {
         mask = (ImageOperandsMask)(mask | ImageOperandsConstOffsetsMask);
         texArgs[numArgs++] = parameters.offsets;