[AArch64] Promote loads from stores

This change promotes load instructions which directly read from stores by
replacing them with mov instructions. If the store is wider than the load,
the load will be replaced with a bitfield extract.
For example :
  STRWui %W1, %X0, 1
  %W0 = LDRHHui %X0, 3
becomes
  STRWui %W1, %X0, 1
  %W0 = UBFMWri %W1, 16, 31

llvm-svn: 256004