Make it possible to specify workgroups as uniform (#734)

* Add option to specify uniform workgroup size
* Error when set as both uniform and non-uniform
* Test uniform workgroup size
diff --git a/lib/Compiler.cpp b/lib/Compiler.cpp
index 361c4a5..92ef79c 100644
--- a/lib/Compiler.cpp
+++ b/lib/Compiler.cpp
@@ -873,6 +873,13 @@
     }
   }
 
+  if (clspv::Option::ArmNonUniformWorkGroupSize() &&
+      clspv::Option::UniformWorkgroupSize()) {
+    llvm::errs() << "cannot enable Arm non-uniform workgroup extension support "
+                    "and assume uniform workgroup sizes\n";
+    return -1;
+  }
+
   return 0;
 }