middle-end: Fix GSI for gcond root [PR117140]

When finding the gsi to use for code of the root statements we should use the
one of the original statement rather than the gcond which may be inside a
pattern.

Without this the emitted instructions may be discarded later.

gcc/ChangeLog:

	PR tree-optimization/117140
	* tree-vect-slp.cc (vectorize_slp_instance_root_stmt): Use gsi from
	original statement.

gcc/testsuite/ChangeLog:

	PR tree-optimization/117140
	* gcc.dg/vect/vect-early-break_129-pr117140.c: New test.