[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