Clean up after transition into opaque pointers. NFC (#102631)

LegacyPointerTypes is not used any longer and can be removed from
the LLVM context.

Also remove a copy-pasted code comment in TypedPointerType that
doesn't make sense (since there is no special case for address space
zero in the TypedPointerType::get implementation).

Author: bjope

llvm/lib/IR/LLVMContextImpl.h

@@ -1608,7 +1608,6 @@ class LLVMContextImpl {
   DenseMap<std::pair<Type *, ElementCount>, VectorType *> VectorTypes;
   PointerType *AS0PointerType = nullptr; // AddrSpace = 0
   DenseMap<unsigned, PointerType *> PointerTypes;
-  DenseMap<std::pair<Type *, unsigned>, PointerType *> LegacyPointerTypes;
   DenseMap<std::pair<Type *, unsigned>, TypedPointerType *> ASTypedPointerTypes;
 
   /// ValueHandles - This map keeps track of all of the value handles that are

llvm/lib/IR/TypedPointerType.cpp

@@ -20,7 +20,6 @@ TypedPointerType *TypedPointerType::get(Type *EltTy, unsigned AddressSpace) {
 
   LLVMContextImpl *CImpl = EltTy->getContext().pImpl;
 
-  // Since AddressSpace #0 is the common case, we special case it.
   TypedPointerType *&Entry =
       CImpl->ASTypedPointerTypes[std::make_pair(EltTy, AddressSpace)];