This patch recognizes (+ (+ v0, v1) (+ v2, v3)), reorders them for bundling into vector of loads,
and vectorizes it.

 Test case :

       float hadd(float* a) {
           return (a[0] + a[1]) + (a[2] + a[3]);
        }


 AArch64 assembly before patch :

        ldp	s0, s1, [x0]
 	ldp	s2, s3, [x0, #8]
 	fadd	s0, s0, s1
 	fadd	s1, s2, s3
 	fadd	s0, s0, s1
 	ret

 AArch64 assembly after patch :

        ldp	d0, d1, [x0]
 	fadd	v0.2s, v0.2s, v1.2s
 	faddp	s0, v0.2s
 	ret

Reviewed Link : http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20141208/248531.html