merge consecutive loads that are offset from a base address

SelectionDAG::isConsecutiveLoad() was not detecting consecutive loads
when the first load was offset from a base address. 

This patch recognizes that pattern and subtracts the offset before comparing
the second load to see if it is consecutive.

The codegen change in the new test case improves from:

vmovsd	32(%rdi), %xmm0
vmovsd	48(%rdi), %xmm1 
vmovhpd	56(%rdi), %xmm1, %xmm1
vmovhpd	40(%rdi), %xmm0, %xmm0
vinsertf128	$1, %xmm1, %ymm0, %ymm0

To:

vmovups	32(%rdi), %ymm0

An existing test case is also improved from:

vmovsd	(%rdi), %xmm0
vmovsd	16(%rdi), %xmm1
vmovsd	24(%rdi), %xmm2
vunpcklpd	%xmm2, %xmm0, %xmm0 ## xmm0 = xmm0[0],xmm2[0]
vmovhpd	8(%rdi), %xmm1, %xmm3

To:

vmovsd	(%rdi), %xmm0
vmovsd	16(%rdi), %xmm1
vmovhpd	24(%rdi), %xmm0, %xmm0
vmovhpd	8(%rdi), %xmm1, %xmm1

This patch fixes PR21771 ( http://llvm.org/bugs/show_bug.cgi?id=21771 ).

Differential Revision: http://reviews.llvm.org/D6642

llvm-svn: 224379