[SCF][Transform] Add transform.loop.fuse_sibling

This patch adds a new transform operation `transform.loop.fuse_sibling`,
which given two loops, fuses them, assuming that they are independent.
The transform operation itself performs very basic checks to ensure
IR legality, and leaves the responsibility of ensuring independence on the user.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D157069