tree-optimization/97623 - limit PRE hoist insertion

tree-optimization/97623 - limit PRE hoist insertion

This limits insert iteration caused by PRE insertions generating
hoist insertion opportunities and vice versa.  The patch limits
the hoist insertion iterations to three by default.

2020-11-03  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97623
	* params.opt (-param=max-pre-hoist-insert-iterations): New.
	* doc/invoke.texi (max-pre-hoist-insert-iterations): Document.
	* tree-ssa-pre.c (insert): Do at most max-pre-hoist-insert-iterations
	hoist insert iterations.