[ARM] Permit VGETLNi32 and VSETLNi32 without mnemonic suffix.

These instructions transfer 32 bits of data between an integer
register and half of a d-register. Currently LLVM accepts them only
with the syntax `vmov.32 r0, d0[0]` or `vmov.32 d0[0], r0`. But the
ARMARM says that the `.32` suffix on the mnemonic should be optional.

Added a pair of NEONInstAlias to accept the bare `vmov` version, and
checked that the result is the same as with `.32`.

This only adds new syntax accepted in assembly. The existing explicit
version is still used when disassembling these instructions.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D156868