Fix potential pointer lifetime issue in the type scavenger. (#1850)

As computerPointerElementType is recursive, it's possible for the hash table to
be resized during the call to the function. If this happens, then the reference
to the value is no longer a valid reference, and may overwrite memory (or just
plain segfault).

Author: jcranmer-intel

lib/SPIRV/SPIRVTypeScavenger.cpp

@@ -293,7 +293,7 @@ SPIRVTypeScavenger::computePointerElementType(Value *V) {
   }
 
   // Check if we've already deduced a type for the value.
-  DeducedType &Ty = DeducedTypes[V];
+  DeducedType Ty = DeducedTypes[V];
   if (Ty) {
     return Ty;
   }
@@ -402,6 +402,7 @@ SPIRVTypeScavenger::computePointerElementType(Value *V) {
     Ty = Deferred;
   }
 
+  DeducedTypes[V] = Ty;
   VisitStack.pop_back();
   return Ty;
 }