ipa: IPA-SRA split detection simplification

I have noticed that the flag m_split_modifications_p of
ipa_param_body_adjustments is not really necessary as it has to
correspond to whether m_replacements is non-empty so this patches
removes it.  This also simplifies a bit some patches I work on.

gcc/ChangeLog:

2022-11-10  Martin Jambor  <mjambor@suse.cz>

	* ipa-param-manipulation.h (ipa_param_body_adjustments): Removed
	member m_split_modifications_p.
	* ipa-param-manipulation.cc
	(ipa_param_body_adjustments::common_initialization): Do not set
	m_split_modifications_p.
	(ipa_param_body_adjustments::ipa_param_body_adjustments): Remove
	initializations of m_split_modifications_p.
	(ipa_param_body_adjustments::modify_call_stmt): Check that
	m_replacements is empty instead of m_split_modifications_p.