sparc: Skip all empty assembly statements

This version detects multiple empty assembly statements in a row and also
detects non-memory barrier empty assembly statements (__asm__("")). It
can be used instead of next_active_insn().

gcc/ChangeLog:

	* config/sparc/sparc.c (next_active_non_empty_insn): New function
	that returns next active non empty assembly instruction.
	(sparc_do_work_around_errata): Use new function.