tree-optimiztation/114855 - profile prediction slowness

The testcase in PR114855 shows profile prediction to evaluate
the same SSA def via expr_expected_value for each condition or
switch in a function.  The following patch caches the expected
value (and probability/predictor) for each visited SSA def,
also protecting against recursion and thus obsoleting the visited
bitmap.  This reduces the time spent in branch prediction from
1.2s to 0.3s, though callgrind which was how I noticed this
seems to be comparatively very much happier about the change than
this number suggests.

	PR tree-optimization/114855
	* predict.cc (ssa_expected_value): New global.
	(expr_expected_value): Do not take bitmap.
	(expr_expected_value_1): Likewise.  Use ssa_expected_value
	to cache results for a SSA def.
	(tree_predict_by_opcode): Adjust.
	(tree_estimate_probability): Manage ssa_expected_value.
	(tree_guess_outgoing_edge_probabilities): Likewise.

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