[AArch64] Improve getUsefulBitsForUse for narrow stores.

For narrow stores (e.g., strb, srth) we know the upper bits of the register are
unused/not useful. In some cases we can use this information to eliminate
unnecessary instructions.

For example, without this patch we generate (from the 2nd test case):

 ldr w8, [x0]
 and w8, w8, #0xfff0
 bfxil w8, w2, #16, #4
 strh w8, [x1]

and after the patch the 'and' is removed:

 ldr w8, [x0]
 bfxil w8, w2, #16, #4
 strh w8, [x1]
 ret

During the lowering of the bitfield insert instruction the 'and' is eliminated
because we know the upper 16-bits that are masked off are unused and the lower
4-bits that are masked off are overwritten by the insert itself. Therefore, the
'and' is unnecessary.

Differential Revision: http://reviews.llvm.org/D20175

llvm-svn: 269226