[LoopInterchange] Enable interchange with multiple inner loop indvars

Currently loop interchange only supports loops with one inner loop
induction variable. This patch adds support for transformation with
more than one inner loop induction variables. The induction PHIs and
induction increment instructions are moved/duplicated properly to the
new outer header and the new outer latch, respectively.

Reviewed By: bmahjour

Differential Revision: https://reviews.llvm.org/D114917