[ARM] GlobalISel: Lower only homogeneous struct args

Lowering mixed struct args, params and returns used G_INSERT, which is a
bit more convoluted to support through the entire pipeline. Since they
don't occur that often in practice, it's probably wiser to leave them
out until later.

Meanwhile, we can lower homogeneous structs using G_MERGE_VALUES, which
has good support in the legalizer. These occur e.g. as the return of
__aeabi_idivmod, so it's nice to be able to support them.

llvm-svn: 305458