Reverts commit r301424, r301425 and r301426

Commits were:

"Use WeakVH instead of WeakTrackingVH in AliasSetTracker's UnkownInsts"
"Add a new WeakVH value handle; NFC"
"Rename WeakVH to WeakTrackingVH; NFC"

The changes assumed pointers are 8 byte aligned on all architectures.

llvm-svn: 301429

llvm/lib/Bitcode/Reader/ValueList.cpp

@@ -58,7 +58,7 @@ void BitcodeReaderValueList::assignValue(Value *V, unsigned Idx) {
   if (Idx >= size())
     resize(Idx + 1);
 
-  WeakTrackingVH &OldV = ValuePtrs[Idx];
+  WeakVH &OldV = ValuePtrs[Idx];
   if (!OldV) {
     OldV = V;
     return;