ipa-sra: Don't consider CLOBBERS as writes preventing splitting

When IPA-SRA detects whether a parameter passed by reference is
written to, it does not special case CLOBBERs which means it often
bails out unnecessarily, especially when dealing with C++ destructors.
Fixed by the obvious continue in the two relevant loops and by adding
a simple function that marks the clobbers in the transformation code
as statements to be removed.

gcc/ChangeLog:

2023-08-04  Martin Jambor  <mjambor@suse.cz>

	PR ipa/110378
	* ipa-param-manipulation.h (class ipa_param_body_adjustments): New
	members get_ddef_if_exists_and_is_used and mark_clobbers_dead.
	* ipa-sra.cc (isra_track_scalar_value_uses): Ignore clobbers.
	(ptr_parm_has_nonarg_uses): Likewise.
	* ipa-param-manipulation.cc
	(ipa_param_body_adjustments::get_ddef_if_exists_and_is_used): New.
	(ipa_param_body_adjustments::mark_dead_statements): Move initial
	checks to get_ddef_if_exists_and_is_used.
	(ipa_param_body_adjustments::mark_clobbers_dead): New.
	(ipa_param_body_adjustments::common_initialization): Call
	mark_clobbers_dead when splitting.

gcc/testsuite/ChangeLog:

2023-07-31  Martin Jambor  <mjambor@suse.cz>

	PR ipa/110378
	* g++.dg/ipa/pr110378-1.C: New test.