[AArch64] Generalize extract-high DUP extension to MOVI/MVNI.

These are really immediate DUPs, and suffer from the same problem
with long instructions with a high/2 variant (e.g. smull).

By extending a MOVI (or DUP, before this patch), we can avoid an ext
on the other operand of the long instruction, e.g. turning:
    ext.16b v0, v0, v0, #8
    movi.4h v1, #0x53
    smull.4s  v0, v0, v1
into:
    movi.8h v1, #0x53
    smull2.4s  v0, v0, v1

While there, add a now-necessary combine to fold (VT NVCAST (VT x)).

llvm-svn: 239799