[AArch64] Combine vector shift instructions in SelectionDAG

bswap.v2i16 + sitofp in LLVM IR generate a sequence of:

  - REV32 + USHR for bswap.v2i16
  - SHL + SSHR + SCVTF for sext to v2i32 and scvt

The shift instructions are excessive as noted in PR24820, and they can
be optimized to just SSHR.

Differential Revision: https://reviews.llvm.org/D102333