Hoist the insertVector helper to be a static helper.

This will allow its use inside of memcpy rewriting as well. This routine
is more complex than extractVector, and some of its uses are not 100%
where I want them to be so there is still some work to do here.

While this can technically change the output in some cases, it shouldn't
be a change that matters -- IE, it can leave some dead code lying around
that prior versions did not, etc.

Yet another step in the refactorings leading up to the solution to the
last component of PR14478.

llvm-svn: 170328