[AArch64]Merge halfword loads into a 32-bit load

[AArch64]Merge halfword loads into a 32-bit load

Convert two halfword loads into a single 32-bit word load with bitfield extract
instructions. For example :
  ldrh w0, [x2]
  ldrh w1, [x2, #2]
becomes
  ldr w0, [x2]
  ubfx w1, w0, #16, #16
  and  w0, w0, #ffff

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250719 91177308-0d34-0410-b5e6-96231b3b80d8