ipa-cp: Fold-convert values when necessary (PR 118138)

PR 118138 and quite a few duplicates that it has acquired in a short
time show that even though we are careful to make sure we do not loose
any bits when newly allowing type conversions in jump-functions, we
still need to perform the fold conversions during IPA constant
propagation and not just at the end in order to properly perform
sign-extensions or zero-extensions as appropriate.

This patch does just that, changing a safety predicate we already use
at the appropriate places to return the necessary type.

gcc/ChangeLog:

2025-01-03  Martin Jambor  <mjambor@suse.cz>

	PR ipa/118138
	* ipa-cp.cc (ipacp_value_safe_for_type): Return the appropriate
	type instead of a bool, accept NULL_TREE VALUEs.
	(propagate_vals_across_arith_jfunc): Use the new returned value of
	ipacp_value_safe_for_type.
	(propagate_vals_across_ancestor): Likewise.
	(propagate_scalar_across_jump_function): Likewise.

gcc/testsuite/ChangeLog:

2025-01-03  Martin Jambor  <mjambor@suse.cz>

	PR ipa/118138
	* gcc.dg/ipa/pr118138.c: New test.