rtl-optimization/110587 - remove quadratic regno_in_use_p

rtl-optimization/110587 - remove quadratic regno_in_use_p

The following removes the code checking whether a noop copy
is between something involved in the return sequence composed
of a SET and USE.  Instead of checking for this special-case
the following makes us only ever remove noop copies between
pseudos - which is the case that is necessary for IRA/LRA
interfacing to function according to the comment.  That makes
looking for the return reg special case unnecessary, reducing
the compile-time in LRA non-specific to zero for the testcase.

	PR rtl-optimization/110587
	* lra-spills.cc (return_regno_p): Remove.
	(regno_in_use_p): Likewise.
	(lra_final_code_change): Do not remove noop moves
	between hard registers.