tree-optimization/101373 - avoid PRE across externally throwing call

tree-optimization/101373 - avoid PRE across externally throwing call

PRE already tries to avoid hoisting possibly trapping expressions
across calls that might not return normally but fails to consider
const calls that throw externally.  The following fixes that and
also plugs the hole of trapping references not pruned in case
they are not catched by the actuall call clobbering it.

At -Os we hit the same issue in RTL PRE and postreload-gcse has
even more incomplete checks so the patch adjusts both of those
as well.

2021-07-08  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101373
	* tree-ssa-pre.c (prune_clobbered_mems): Also prune trapping
	references when the BB may not return.
	(compute_avail): Pass in the function we're working on and
	replace cfun references with it.  Externally throwing
	const calls also possibly terminate the function.
	(pass_pre::execute): Pass down the function we're working on.
	* gcse.c (compute_hash_table_work): Externally throwing
	const/pure calls also need record_last_mem_set_info.
	* postreload-gcse.c (record_opr_changes): Looping or externally
	throwing const/pure calls also need record_last_mem_set_info.

	* g++.dg/torture/pr101373.C: New testcase, XFAILed.
	* gnat.dg/opt95.adb: Likewise.