Fix segfault with atomic arg check

Makes sure that we have an l-value before checking the storage type of
the mem argument passed to an atomic memory operation.

Fixes #3332.

Test/spv.atomicRvalue.error.vert

@@ -0,0 +1,7 @@
+#version 440
+
+void main() {
+    uint a = 5;
+    atomicAdd(a * 2, 0);
+    atomicAdd(a, 0);
+}