[msan] Fix __msan_maybe_ for non-standard type sizes.

Fix incorrect calculation of the type size for __msan_maybe_warning_N
call that resulted in an invalid (narrowing) zext instruction and
"Assertion `castIsValid(op, S, Ty) && "Invalid cast!"' failed."

Only happens in very large functions (with more than 3500 MSan
checks) operating on integer types that are not power-of-two.

llvm-svn: 274395