SpecOp bool->uint/int and uint<->int conversion

Bool -> uint/int with OpSpecConstantOp OpSelect instruction.

uint <-> int conversion with OpSpecConstantOp OpIAdd instruction.

Note, implicit conversion: `const uint = an_int_spec_constant` is not
supported. Explicit type casting is required: `const uint =
uint(an_int_spec_constant)`
diff --git a/SPIRV/GlslangToSpv.cpp b/SPIRV/GlslangToSpv.cpp
index 9249574..0240d8d 100755
--- a/SPIRV/GlslangToSpv.cpp
+++ b/SPIRV/GlslangToSpv.cpp
@@ -1960,7 +1960,6 @@
     glslang::TIntermTyped* specNode = arraySizes.getDimNode(dim);
     if (specNode != nullptr) {
         builder.clearAccessChain();
-        // SpecConstantOpModeGuard set_to_spec_const_mode(&builder);
         specNode->traverse(this);
         return accessChainLoad(specNode->getAsTyped()->getType());
     }
@@ -3307,7 +3306,27 @@
         break;
 
     case glslang::EOpConvUintToInt:
+        if (builder.isInSpecConstCodeGenMode()) {
+            // Build zero scalar or vector for OpIAdd to do the conversion when
+            // generating for OpSpecConstantOp instruction.
+            zero = builder.makeIntConstant(0);
+            zero = makeSmearedConstant(zero, vectorSize);
+        }
+        // Don't 'break' here as this case should be grouped together with
+        // EOpConvIntToUint when generating normal run-time conversion
+        // instruction.
     case glslang::EOpConvIntToUint:
+        if (builder.isInSpecConstCodeGenMode()) {
+            // Build zero scalar or vector for OpIAdd.
+            if (zero == 0) {
+                zero = builder.makeUintConstant(0);
+                zero = makeSmearedConstant(zero, vectorSize);
+            }
+            // Use OpIAdd, instead of OpBitcast to do the conversion when
+            // generating for OpSpecConstantOp instruction.
+            return builder.createBinOp(spv::OpIAdd, destType, operand, zero);
+        }
+        // For normal run-time conversion instruction, use OpBitcast.
         convOp = spv::OpBitcast;
         break;