Merge consecutive 16-byte loads into one 32-byte load (PR22329)

This patch detects consecutive vector loads using the existing 
EltsFromConsecutiveLoads() logic. This fixes:
http://llvm.org/bugs/show_bug.cgi?id=22329

This patch effectively reverts the tablegen additions of D6492 / 
http://reviews.llvm.org/rL224344 ...which in hindsight were a horrible hack.

The test cases that were added with that patch are simply modified to load
from varying offsets of a base pointer. These loads did not match the existing
tablegen patterns.

A happy side effect of doing this optimization earlier is that we can now fold
the load into a math op where possible; this is shown in some of the updated
checks in the test file.

Differential Revision: http://reviews.llvm.org/D7303

llvm-svn: 228006