VECT: Support loop len control on EXTRACT_LAST vectorization

VECT: Support loop len control on EXTRACT_LAST vectorization

Hi, @Richi and @Richard, base on previous disscussion, I simpily fix issuses for
powerpc and s390 with your suggestions:

-  machine_mode len_load_mode = get_len_load_store_mode
-    (loop_vinfo->vector_mode, true).require ();
-  machine_mode len_store_mode = get_len_load_store_mode
-    (loop_vinfo->vector_mode, false).require ();
+  machine_mode len_load_mode, len_store_mode;
+  if (!get_len_load_store_mode (loop_vinfo->vector_mode, true)
+        .exists (&len_load_mode))
+    return false;
+  if (!get_len_load_store_mode (loop_vinfo->vector_mode, false)
+        .exists (&len_store_mode))
+    return false;

Co-Authored-By: Kewen.Lin <linkw@linux.ibm.com>
gcc/ChangeLog:

	* tree-vect-loop.cc (vect_verify_loop_lens): Add exists check.
	(vectorizable_live_operation): Add live vectorization for length loop
	control.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/partial/live-1.c: New test.
	* gcc.target/riscv/rvv/autovec/partial/live_run-1.c: New test.