Lift the extractVector helper all the way out to a static helper function.

The method helpers all implicitly act upon the alloca, and what we
really want is a fully generic helper. Doing memcpy rewrites is more
special than all other rewrites because we are at times rewriting
instructions which touch pointers *other* than the alloca. As
a consequence all of the helpers needed by memcpy rewriting of
sub-vector copies will need to be generalized fully.

Note that all of these helpers ({insert,extract}{Integer,Vector}) are
woefully uncommented. I'm going to go back through and document them
once I get the factoring correct.

No functionality changed.

llvm-svn: 170325