s390x's C ABI ty_align and ty_size are not moved because the
implementation of ty_align varies in an atypical pattern: it calls
ty_size for the llvm::Vector type kind. ty_size then cannot be moved
since it indirectly calls ty_align through align.
We need to supply sext/zext attributes to LLVM to ensure that arguments
are extended to the appropriate width in the correct way.
Most platforms extend integers less than 32 bits, though not all.
