ipa: Avoid looking for IPA-SRA replacements where there are none

While modifying the code, I realized that we do look into statements
even when there are no replacements.  This patch adds the necessary
early bail-outs to avoid that.

ipa_param_body_adjustments::modify_call_stmt cannot have the same at
the very beginning because calls can still contain otherwise removed
parameters that need to be removed from the statements too.

gcc/ChangeLog:

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

	* ipa-param-manipulation.cc
	(ipa_param_body_adjustments::modify_expression): Bail out early if
	there are no replacements.
	(ipa_param_body_adjustments::modify_assignment): Likewise.