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.

gtests/Spv.FromFile.cpp

@@ -529,6 +529,7 @@ INSTANTIATE_TEST_SUITE_P(
         "spv.ext.texture_shadow_lod.frag",
         "spv.ext.texture_shadow_lod.error.frag",
         "spv.floatFetch.frag",
+        "spv.atomicRvalue.error.vert",
     })),
     FileNameAsCustomTestSuffix
 );