[AArch64][SelectionDAG] Lower multiplication by a constant to shl+add+shl+add

Change the costmodel to lower a = b * C where C = (1 + 2^m) * (1 + 2^n) to
      add   w8, w0, w0, lsl #m
      add   w0, w8, w8, lsl #n
Note: The latency can vary depending on the shirt amount

Reviewed By: efriedma, dmgreen
Differential Revision: https://reviews.llvm.org/D135441