[AArch64] Reuse larger DUP if available

If both a v2i32 DUP(x) and a v4i32 DUP(x) node exists, we can re-use the
larger node using a vector extract to obtain the smaller. This comes up
in the smull/smlal code, but needs a small fixup to allow the smull2
code in tryExtendDUPToExtractHigh/performAddSubLongCombine to still
match smull2 extracts.

Differential Revision: https://reviews.llvm.org/D126449