ipa-sra: Do not remove return values needed because of non-call EH

ipa-sra: Do not remove return values needed because of non-call EH

IPA-SRA already contains a check to figure out that an otherwise dead
parameter is actually required because of non-call exceptions, but it
is not present at the equivalent spot where SRA figures out whether
the return statement is used for anything useful.  This patch adds
that condition there.

Unfortunately, even though this patch should be good enough for any
normal (I'd even say reasonable) use of the compiler, it hints that
when the user manually switches all sorts of DCE, IPA-SRA would
probably leave behind problematic statements manipulating what
originally were return values, just like it does for parameters (PR
93385).  Fixing this properly might unfortunately be a separate issue
from the mentioned bug because the LHS of a call is changed during
call redirection and the caller often is not a clone.  But I'll see
what I can do.

Meanwhile, the patch below has been bootstrapped and tested on x86_64.

gcc/ChangeLog:

2021-01-18  Martin Jambor  <mjambor@suse.cz>

	PR ipa/98690
	* ipa-sra.c (ssa_name_only_returned_p): New parameter fun.  Check
	whether non-call exceptions allow removal of a statement.
	(isra_analyze_call): Pass the appropriate function to
	ssa_name_only_returned_p.

gcc/testsuite/ChangeLog:

2021-01-18  Martin Jambor  <mjambor@suse.cz>

	PR ipa/98690
	* g++.dg/ipa/pr98690.C: New test.