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

This patch is a step 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 r0, r1, d16
but it does not understand yet
vmov.32 d16[0], r0
vmov.32 d16[1], r1

Comming patches will fix that and update the related test case.

<rdar://problem/12702965>