[MemCpyOpt] Don't force i64 when promoting memset/memcpy sizes.

Harden r235258 to support any integer bitwidth.  The quick glance at
the reference made me think only i32 and i64 were valid types, but
they're not special, so any overload is legal.

Thanks to David Majnemer for noticing!

llvm-svn: 235261