[AArch64] Convert vector add(ext, ext) into ext(add(ext, ext))

Given a vector add or sub from extends that needs more that one 'step'
(i.e i8 to i32 or i16 to i64), we can transform the sequence to
sext(add(ext, ext)), to allow the add(ext, ext) to become a single uaddl
and a larger extend, producing less instructions in total.
https://alive2.llvm.org/ce/z/S2T4k-

Differential Revision: https://reviews.llvm.org/D128426