This is the first step to support software pipeline for scf.for loops.
This is only the transformation to create pipelined kernel and
prologue/epilogue.
The scheduling needs to be given by user as many different algorithm
and heuristic could be applied.
This currently doesn't handle loop arguments, this will be added in a
follow up patch.
Differential Revision: https://reviews.llvm.org/D105868
thomasraoux