tree-optimization/97623 - avoid excessive insert iteration for hoisting

tree-optimization/97623 - avoid excessive insert iteration for hoisting

This avoids requiring insert iteration for back-to-back hoisting
opportunities as seen in the added testcase.  For the PR at hand
this halves the number of insert iterations retaining only
the hard to avoid PRE / hoist insert back-to-backs.

2020-10-30  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97623
	* tree-ssa-pre.c (insert): First do hoist insertion in
	a backward walk.

	* gcc.dg/tree-ssa/ssa-hoist-7.c: New testcase.