expand: empty class return optimization [PR88529]

expand: empty class return optimization [PR88529]

The x86_64 psABI says that an empty class isn't passed or returned in memory
or registers, so we shouldn't set %eax in this function.

The df-scan hunk catches the case where we look at a 0-length reg and build
a range the length of unsigned int, which happened before I changed
assign_parms to match expand_function_end.

	PR target/88529

gcc/ChangeLog:

	* df-scan.c (df_ref_record): Check that regno < endregno.
	* function.c (assign_parms, expand_function_end): Do nothing with a
	TYPE_EMPTY_P result.

gcc/testsuite/ChangeLog:

	* g++.target/i386/empty-class1.C: New test.