Refactor insert and extract of sub-integers into static helpers that

operate purely on values. Sink the alloca loading and storing logic into
the rewrite routines that are specific to alloca-integer-rewrite
driving. This is just a refactoring here, but the subsequent step will
be to reuse the insertion and extraction logic when rewriting integer
loads and stores that have been split and decomposed into narrower loads
and stores.

No functionality changed other than different names for instructions.

llvm-svn: 166176