[LoopInterchange] Enable interchange with multiple outer loop indvars

This patch enables loop interchange with multiple outer loop
induction variables, and hence removes the limitation that only
a single outer loop induction variable is supported. In fact, it
turns out that the current pass already trivially supports multiple
outer indvars, which is the result of a previous patch
`https://reviews.llvm.org/D102743`. Therefore, this patch removed that
limitation and provides test cases for multiple outer indvars.

Reviewed By: bmahjour

Differential Revision: https://reviews.llvm.org/D114916