[AArch64] Generate a BFI/BFXIL from 'or (and X, MaskImm), OrImm'.

If and only if the value being inserted sets only known zero bits.

This combine transforms things like

  and w8, w0, #0xfffffff0
  movz w9, #5
  orr w0, w8, w9

into

  movz w8, #5
  bfxil w0, w8, #0, #4

The combine is tuned to make sure we always reduce the number of instructions.
We avoid churning code for what is expected to be performance neutral changes
(e.g., converted AND+OR to OR+BFI).

Differential Revision: http://reviews.llvm.org/D20387

llvm-svn: 270846