[PeepholeOptimizer] Take advantage of the isInsertSubreg property in the
advanced copy optimization.

This is the final step patch toward transforming:
udiv    r0, r0, r2
udiv    r1, r1, r3
vmov.32 d16[0], r0
vmov.32 d16[1], r1
vmov    r0, r1, d16
bx      lr

into:
udiv    r0, r0, r2
udiv    r1, r1, r3
bx      lr

Indeed, thanks to this patch, this optimization is able to look through
vmov.32 d16[0], r0
vmov.32 d16[1], r1

and is able to rewrite the following sequence:
vmov.32 d16[0], r0
vmov.32 d16[1], r1
vmov    r0, r1, d16

into simple generic GPR copies that the coalescer managed to remove.

<rdar://problem/12702965>