Add an assert that ID operands are non-zero

Zero is not a valid ID value and the SPIR-V emitter library should never
be emitting instructions with ID values of 0.

SPIRV/spvIR.h

@@ -97,6 +97,8 @@ public:
     explicit Instruction(Op opCode) : resultId(NoResult), typeId(NoType), opCode(opCode), block(nullptr) { }
     virtual ~Instruction() {}
     void addIdOperand(Id id) {
+        // ids can't be 0
+        assert(id);
         operands.push_back(id);
         idOperand.push_back(true);
     }