ipa/109652 - ICE in modification phase of IPA SRA

ipa/109652 - ICE in modification phase of IPA SRA

There's another questionable IL transform by IPA SRA, replacing
foo (p_1(D)->x) with foo (VIEW_CONVERT <union type> (ISRA.PARM.1))
where ISRA.PARM.1 is a register.  Conversion of a register to
an aggregate type is questionable but not entirely unreasonable
and not within the set of IL I am rejecting when fixing PR109644.

The following lets this slip through in IPA SRA transform by
restricting re-gimplification to the case of register type
results.  To not break the previous testcase again we need to
optimize the BIT_FIELD_REF <VIEW_CONVERT <...>, ...> case
to elide the conversion.

	PR ipa/109652
	* ipa-param-manipulation.cc
	(ipa_param_body_adjustments::modify_expression): Allow
	conversion of a register to a non-register type.  Elide
	conversions inside BIT_FIELD_REFs.

	* gcc.dg/torture/pr109652.c: New testcase.