Fix a major source of compile-time slowness at -O0 -g by optimizing

the storage of !dbg metadata kinds in the instruction themselves.
The on-the-side hash table works great for metadata that not-all
instructions get, or for metadata that only exists when optimizing.
But when compile-time is everything, it isn't great.

I'm not super thrilled with the fact that this plops a TrackingVH in
Instruction, because it grows it by 3 words.  I'm investigating 
alternatives, but this should be a step in the right direction in any
case.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@99957 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/ValueHandle.h

@@ -302,7 +302,7 @@ class TrackingVH : public ValueHandleBase {
 
   ValueTy *getValPtr() const {
     CheckValidity();
-    return static_cast<ValueTy*>(ValueHandleBase::getValPtr());
+    return (ValueTy*)ValueHandleBase::getValPtr();
   }
   void setValPtr(ValueTy *P) {
     CheckValidity();