Introduce REG_SET_EMPTY_P

Introduce REG_SET_EMPTY_P

This avoids a -Wunreachable-code diagnostic with EXECUTE_IF_*
in case the first iteration will exit the loop.  For the case
in thread_jump using bitmap_empty_p looks preferable so this
adds REG_SET_EMPTY_P to make that available for register sets.

2021-11-25  Richard Biener  <rguenther@suse.de>

	* regset.h (REG_SET_EMPTY_P): New macro.
	* cfgcleanup.c (thread_jump): Use REG_SET_EMPTY_P.