ipa/109607 - properly gimplify conversions introduced by IPA param manipulation

ipa/109607 - properly gimplify conversions introduced by IPA param manipulation

The following addresses IPA param manipulation (through IPA SRA)
replacing

  BIT_FIELD_REF <*this_8(D), 8, 56>

with

  BIT_FIELD_REF <VIEW_CONVERT_EXPR<const struct profile_count>(ISRA.814), 8, 56>

which is supposed to be invalid GIMPLE (ISRA.814 is a register).
There's currently insufficient checking in place to catch this in the
IL verifier but I am working on that as part of fixing PR109594.

The solution for the particular testcase I am running into this is
to split the conversion to a separate stmt.  Generally the modification
phase is set up for this but the extra_stmts sequence isn't passed
around everywhere.  The following passes it to modify_expression
from modify_assignment when rewriting the RHS.

	PR ipa/109607
	* ipa-param-manipulation.h
	(ipa_param_body_adjustments::modify_expression): Add extra_stmts
	argument.
	* ipa-param-manipulation.cc
	(ipa_param_body_adjustments::modify_expression): Likewise.
	When we need a conversion and the replacement is a register
	split the conversion out.
	(ipa_param_body_adjustments::modify_assignment): Pass
	extra_stmts to RHS modify_expression.

	* g++.dg/torture/pr109607.C: New testcase.