rtl-ssa: Fix updates to call clobbers [PR98403]

rtl-ssa: Fix updates to call clobbers [PR98403]

In the PR, fwprop was changing a call instruction and tripped
an assert when trying to update a list of call clobbers.
There are two ways we could handle this: remove the call clobber
and then add it back, or assume that the clobber will stay in its
current place.

At the moment we don't have enough information to safely move
calls around, so the second approach seems simpler and more
efficient.

gcc/
	PR rtl-optimization/98403
	* rtl-ssa/changes.cc (function_info::finalize_new_accesses): Explain
	why we don't remove call clobbers.
	(function_info::apply_changes_to_insn): Don't attempt to add
	call clobbers here.

gcc/testsuite/
	PR rtl-optimization/98403
	* g++.dg/opt/pr98403.C: New test.