indvars: Code reorganization in preparation for

LinearFunctionTestReplace rewrite. No functionality.

I've been wanting to group the indvar subphases into sections and
order them by their logical sequence. My next checkin adds functions
related to LFTR, and doing the reorg now should help reviewers. Since,
most of the code in IndVarSimplify.cpp has recently been replaced or
will be replaced soon, obscuring blame should not be an issue. This
seems like an ideal time to shuffle the code around.

I'm happy to take more suggestions for cleaning up the code. Or if
you've been wanting to cleanup anything in this file yourself, now is
a good time.

llvm-svn: 134941