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/baseResults/atomicAdd.comp.out

@@ -1,5 +1,5 @@
 atomicAdd.comp
-ERROR: 0:18: 'atomicAdd' : Atomic memory function can only be used for shader storage block member or shared variable. 
+ERROR: 0:18: 'atomicAdd' : Only l-values corresponding to shader block storage or shared variables can be used with atomic memory functions. 
 ERROR: 1 compilation errors.  No code generated.
 
 

Test/baseResults/spv.atomicRvalue.error.vert.out

@@ -0,0 +1,9 @@
+spv.atomicRvalue.error.vert
+ERROR: 0:5: 'assign' :  l-value required 
+ERROR: 0:5: 'out' : Non-L-value cannot be passed for 'out' or 'inout' parameters. 
+ERROR: 0:5: 'atomicAdd' : Only l-values corresponding to shader block storage or shared variables can be used with atomic memory functions. 
+ERROR: 0:6: 'atomicAdd' : Only l-values corresponding to shader block storage or shared variables can be used with atomic memory functions. 
+ERROR: 4 compilation errors.  No code generated.
+
+
+SPIR-V is not generated for failed compile or link

Test/spv.atomicRvalue.error.vert

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