Avoid changing PHIs in GIMPLE split_edge

Avoid changing PHIs in GIMPLE split_edge

Previously I've changed gimple_split_edge to avoid PHI node
re-allocation, but this introduced swapping of PHI arguments
due to the way edge redirection works.  This is now a problem
for me and which can be solved with the following approach
reducing the overhead of split_edge even more.  We can simply
pretend there are no PHI nodes if we can make sure the
new fallthru will have the same dest_idx as the old edge
into the destination.

2020-10-20  Richard Biener  <rguenther@suse.de>

	* tree-cfg.c (reinstall_phi_args): Remove.
	(gimple_split_edge): Remove PHIs around the edge redirection
	to avoid touching them at all.