REE: PR rtl-optimization/100264: Handle more PARALLEL SET expressions

REE: PR rtl-optimization/100264: Handle more PARALLEL SET expressions

Move the check for register targets (i.e. REG_P ()) into the function
get_sub_rtx () and change the restriction of REE to "only one child of
a PARALLEL expression is a SET register expression" (was "only one child of
a PARALLEL expression is a SET expression").

This allows to handle more PARALLEL SET expressions.

gcc/ChangeLog:
	PR rtl-optimization/100264
	* ree.c (get_sub_rtx): Ignore SET expressions without register
	destinations and remove assertion, as it is not valid anymore
	with this new behaviour.
	(merge_def_and_ext): Eliminate destination check for register
	as such SET expressions can't occur anymore.
	(combine_reaching_defs): Likewise.