Fix the first part of PR14478: memset now works.

PR14478 highlights a serious problem in SROA that simply wasn't being
exercised due to a lack of vector input code mixed with C-library
function calls. Part of SROA was written carefully to handle subvector
accesses via memset and memcpy, but the rewriter never grew support for
this. Fixing it required refactoring the subvector access code in other
parts of SROA so it could be shared, and then fixing the splat formation
logic and using subvector insertion (this patch).

The PR isn't quite fixed yet, as memcpy is still broken in the same way.
I'm starting on that series of patches now.

Hopefully this will be enough to bring the bullet benchmark back to life
with the bb-vectorizer enabled, but that may require fixing memcpy as
well.

llvm-svn: 170301