reader/spirv: Handle parser error

MaybeEmitCombinatorialValue() didn't check that MakeOperand() or RectifySecondOperandSignedness() didn't error, leading to ICEs.

Bug: crbug.com/tint/804
Change-Id: Ic78487a70a591e718c7b5936c6678e7a19c4b626
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51927
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>

src/reader/spirv/function.cc

@@ -3510,6 +3510,9 @@ TypedExpression FunctionEmitter::MaybeEmitCombinatorialValue(
     auto arg0 = MakeOperand(inst, 0);
     auto arg1 = parser_impl_.RectifySecondOperandSignedness(
         inst, arg0.type, MakeOperand(inst, 1));
+    if (!arg0 || !arg1) {
+      return {};
+    }
     auto* binary_expr = create<ast::BinaryExpression>(Source{}, binary_op,
                                                       arg0.expr, arg1.expr);
     TypedExpression result{ast_type, binary_expr};