[ARM] Fix a check in vmov/vmvn immediate parsing

Summary:
Currently the check is incorrect and the following invalid
instruction is accepted and incorrectly assembled:

  vmov.i32        d2, #0x00a500a6

This patch fixes the issue.

Reviewers: olista01, rengolin

Reviewed By: rengolin

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

Differential Revision: https://reviews.llvm.org/D44460

llvm-svn: 327704