Hoist the rest of the logic for promoting single-store allocas into the

Hoist the rest of the logic for promoting single-store allocas into the
helper function. This leaves both trivial cases handled entirely in
helper functions and merely manages the list of allocas to process in
the run method.

The next step will be to handle all of the trivial promotion work prior
to even creating the core class and the subsequent simplifications that
enables.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186784 91177308-0d34-0410-b5e6-96231b3b80d8