vect: release defs of removed statement

vect: release defs of removed statement

While trying to add use of simple_dce_from_worklist
to the vectorizer so we don't need to run a full blown
DCE pass after the vectorizer, there was a crash noticed
due to a ssa name which has a stmt without a bb. This was
due to not calling release_defs after the call to gsi_remove.

Note the code to remove zero use statements should be able to
remove once the use of simple_dce_from_worklist has been added.
But in the meantime, fixing this bug will also improve memory
usage and a few other things which look through all ssa names.

gcc/ChangeLog:

	* tree-vect-loop.cc (optimize_mask_stores): Call release_defs
	after the call to gsi_remove with last argument of true.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>