[SFN] start rtl block with label, then markers

[SFN] start rtl block with label, then markers

Emitting markers before labels turned out to not be worth the trouble.
The markers outside BBs confuse the ebb scheduler, and they don't add
any useful information.  I'll arrange for markers to be moved past
labels, even in gimple, but for now this will fix the two remaining
known problems on ia64.

for  gcc/ChangeLog

	PR bootstrap/83396
	* cfgexpand.c (expand_gimple_basic_block): Expand label first,
	even if there are markers before it.
	* cfgrtl.c (rtl_verify_bb_layout): Reject DEBUG_INSNs outside BBs.

From-SVN: r255833