tree-optimization/116907 - stale BLOCK reference from DECL_VALUE_EXPR

When we remove unused BLOCKs we fail to clean references to them
from DECL_VALUE_EXPRs of variables in other BLOCKs which in the
PR causes LTO streaming to walk into pointers to GGC freed blocks.

There's the question of whether such DECL_VALUE_EXPRs should keep
variables and blocks referenced live (it doesn't seem to do that)
and whether such DECL_VALUE_EXPRs should have survived in the
first place.

	PR tree-optimization/116907
	* tree-ssa-live.cc (clear_unused_block_pointer_in_block): New
	helper.
	(clear_unused_block_pointer): Call it.