vect: Set pattern_stmt_p on the newly created stmt_vec_info

While adding simple_dce_worklist to the vectorizer, there was a regression
due to the slp patterns would create a SSA name but never free it even if it
never existed in the IR (this case as addsub but complex ones had the same issue).
The reason why it was never freed was the stmt_vec_info was not marked as a pattern stmt,
unlike the other pattern stmts that use vect_init_pattern_stmt instead of vec_info::add_pattern_stmt
(which is used for SLP patterns).

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-vectorizer.cc (vec_info::add_pattern_stmt): Set pattern_stmt_p.

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