[AArch64] Promote loads from stored

This is a recommit of r256004 which was reverted in r256160. The issue was the
incorrect promotion for half and byte loads transformed into mov instructions.
This fix will replace half and byte type loads only with bit field extracts.

Original commit message:

This change promotes load instructions which directly read from stored 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: 256249