[CodeGen][ExpandMemcmp] Allow memcmp to expand to vector loads (2).

 - Targets that want to support memcmp expansions now return the list of
   supported load sizes.
 - Expansion codegen does not assume that all power-of-two load sizes
   smaller than the max load size are valid. For examples, this is not the
   case for x86(32bit)+sse2.

Fixes PR34887.

llvm-svn: 316905