[AArch64]Extend merging narrow loads into a wider load

This change extends r251438 to handle more narrow load promotions
including byte type, unscaled, and signed. For example, this change will
convert :
  ldursh w1, [x0, #-2]
  ldurh  w2, [x0, #-4]
into
  ldur  w2, [x0, #-4]
  asr   w1, w2, #16
  and   w2, w2, #0xffff

llvm-svn: 253577