Avoid PRE insert iteration when possible

The following make sure to only iterate PRE insertion when
necessary - which is when AVAIL_OUT of a predecessor of a
block we already visited changed (that's backedge destinations).

To not regress this also makes sure to locally iterate insertion
since even topological sort of expressions isn't enough to
guarantee we get all opportunities of a block in one iteration.
This avoids costly re-compute of the topologically sorted expression
array (more micro-optimization is possible here).

2020-11-12  Richard Biener  <rguenther@suse.de>

	* tree-ssa-pre.c (bitmap_value_replace_in_set): Return
	whether we have changed anything.
	(do_pre_regular_insertion): Get topologically sorted array
	of expressions from caller.
	(do_pre_partial_partial_insertion): Likewise.
	(insert): Compute topologically sorted arrays of expressions
	here and locally iterate actual insertion.  Iterate only
	when AVAIL_OUT of an already visited block source changed.

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