[AArch64]Extend merging narrow loads into a wider load

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253577 91177308-0d34-0410-b5e6-96231b3b80d8