[naga] Add new function, GuardedIndex::from_expression.

Pull out the code to build a `naga::proc::index::GuardedIndex` from a
`Handle<Expression>` into its own function,
`GuardedIndex::from_expression`. Use that function in
`GuardedIndex::try_resolve_to_constant`.

naga/src/proc/index.rs

@@ -340,12 +340,21 @@ impl GuardedIndex {
         module: &crate::Module,
     ) {
         if let GuardedIndex::Expression(expr) = *self {
-            if let Ok(value) = module
-                .to_ctx()
-                .eval_expr_to_u32_from(expr, &function.expressions)
-            {
-                *self = GuardedIndex::Known(value);
-            }
+            *self = GuardedIndex::from_expression(expr, function, module);
+        }
+    }
+
+    pub(crate) fn from_expression(
+        expr: Handle<crate::Expression>,
+        function: &crate::Function,
+        module: &crate::Module,
+    ) -> Self {
+        match module
+            .to_ctx()
+            .eval_expr_to_u32_from(expr, &function.expressions)
+        {
+            Ok(value) => Self::Known(value),
+            Err(_) => Self::Expression(expr),
         }
     }
 }