[PR99422] LRA: Don't check unknown constraint, use X for empty constraint

[PR99422] LRA: Don't check unknown constraint, use X for empty constraint

Using CONSTRAINT__UNKNOWN was a bad idea, although it triggered a lot
hidden bugs.  It is better to use X instead of empty constraint.

gcc/ChangeLog:

	PR target/99422
	* lra-constraints.c (process_address_1): Don't check unknown
	constraint, use X for empty constraint.