middle-end: fix ICE when moving statements to empty BB [PR113731]

middle-end: fix ICE when moving statements to empty BB [PR113731]

We use gsi_move_before (&stmt_gsi, &dest_gsi); to request that the new statement
be placed before any other statement.  Typically this then moves the current
pointer to be after the statement we just inserted.

However it looks like when the BB is empty, this does not happen and the CUR
pointer stays NULL.   There's a comment in the source of gsi_insert_before that
explains:

/* If CUR is NULL, we link at the end of the sequence (this case happens

This adds a default parameter to gsi_move_before to allow us to control where
the insertion happens.

gcc/ChangeLog:

	PR tree-optimization/113731
	* gimple-iterator.cc (gsi_move_before): Take new parameter for update
	method.
	* gimple-iterator.h (gsi_move_before): Default new param to
	GSI_SAME_STMT.
	* tree-vect-loop.cc (move_early_exit_stmts): Call gsi_move_before with
	GSI_NEW_STMT.

gcc/testsuite/ChangeLog:

	PR tree-optimization/113731
	* gcc.dg/vect/vect-early-break_111-pr113731.c: New test.