Merge vector stores into wider vector stores (PR21711)

This patch resolves part of PR21711 ( http://llvm.org/bugs/show_bug.cgi?id=21711 ).

The 'f3' test case in that report presents a situation where we have two 128-bit
stores extracted from a 256-bit source vector. 

Instead of producing this:

vmovaps %xmm0, (%rdi)
vextractf128    $1, %ymm0, 16(%rdi)

This patch merges the 128-bit stores into a single 256-bit store:

vmovups %ymm0, (%rdi)

Differential Revision: http://reviews.llvm.org/D7208

llvm-svn: 227242