SPIR-V: Return undefined values from implicit returns instead of dummy

Previously if a non-void function implictly returned, a dummy variable
was created as return value. Now instead it returns the result of the
OpUndef instruction. This better conveys the presence of undefined
behavior to SPIR-V consuming tools (and humans).

It also saves one ID per occurrence...

SPIRV/SpvBuilder.h

@@ -218,6 +218,9 @@ public:
     // Create a global or function local or IO variable.
     Id createVariable(StorageClass, Id type, const char* name = 0);
 
+    // Create an imtermediate with an undefined value.
+    Id createUndefined(Id type);
+
     // Store into an Id and return the l-value
     void createStore(Id rValue, Id lValue);