[MSVC Compat] Don't provide /volatile:ms semantics to types > pointer

Volatile loads of type wider than a pointer get split by MSVC because
the base x86 ISA doesn't provide loads which are wider than pointer
width.  LLVM assumes that it can emit an cmpxchg8b but this is
problematic if the memory is in a CONST memory segment.

Instead, provide behavior compatible with MSVC: split loads wider than a
pointer.

llvm-svn: 258506