[ARM] Convert more invalid NEON immediate loads

Summary:
Currently the LLVM MC assembler is able to convert e.g.

  vmov.i32 d0, #0xabababab

(which is technically invalid) into a valid instruction

  vmov.i8 d0, #0xab

this patch adds support for vmov.i64 and for cases with the resulting
load types other than i8, e.g.:

  vmov.i32 d0, #0xab00ab00 ->
  vmov.i16 d0, #0xab00

Reviewers: olista01, rengolin

Reviewed By: rengolin

Subscribers: rengolin, javed.absar, kristof.beyls, rogfer01, llvm-commits

Differential Revision: https://reviews.llvm.org/D44467

llvm-svn: 327709