[AArch64] Don't force MVT::Untyped when selecting LD1LANEpost.

The result is either an Untyped reg sequence, on ldN with N > 1, or
just the type of the input vector, on ld1.  Don't force Untyped.
Instead, just use the type of the reg sequence.

This mirrors the behavior of createTuple, which feeds the LD1*_POST.

The narrow code path wasn't actually covered by tests, because V64
insert_vector_elt are widened to V128 before the LD1LANEpost combine
has the chance to run, usually.

The only case where it does run on V64 vectors is if the vector ops
legalizer ran.  So, tickle the code with a ctpop.

Fixes PR23265.

llvm-svn: 235243