tree-optimization/102943 - avoid (re-)computing dominance bitmap

Currently back_propagate_equivalences tries to optimize dominance
queries in a smart way but it fails to notice that when fast indexes
are available the dominance query is fast (when called from DOM).
It also re-computes the dominance bitmap for each equivalence recorded
on an edge, which for FP are usually several.  Finally it fails to
use the tree bitmap view for efficiency.  Overall this cuts 7
seconds of compile-time from originally 77 in the slowest LTRANS
unit when building 521.wrf_r.

2022-03-10  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/102943
	* tree-ssa-dom.cc (back_propagate_equivalences): Only
	populate the dominance bitmap if fast queries are not
	available.  Use a tree view bitmap.
	(record_temporary_equivalences): Cache the dominance bitmap
	across all equivalences on the edge.

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