VECT: Fix bug of multiple-rgroup for length is counting elements

VECT: Fix bug of multiple-rgroup for length is counting elements

Address comments from Richard that splits the patch of fixing
multiple-rgroup
handling of length counting elements.

This patch is fixing issue of handling multiple-rgroup of length is
counting elements

Before this patch, multiple rgroup run fail:
FAIL: gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-1.c
execution test
FAIL: gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-2.c
execution test
FAIL: gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-1.c
execution test
FAIL: gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-2.c
execution test
FAIL: gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-1.c
execution test
FAIL: gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-2.c
execution test
FAIL: gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-2.c
execution test
FAIL: gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-1.c
execution test
FAIL: gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-1.c
execution test
FAIL: gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-2.c
execution test
FAIL: gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-1.c
execution test
FAIL: gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-2.c
execution test
FAIL: gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-1.c
execution test

After this patch, These tests are all passed.

gcc/ChangeLog:

	* tree-vect-loop.cc (vect_get_loop_len): Fix issue for
	multiple-rgroup of length.
	* tree-vect-stmts.cc (vectorizable_store): Ditto.
	(vectorizable_load): Ditto.
	* tree-vectorizer.h (vect_get_loop_len): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/partial/multiple_rgroup-1.c: New
	test.
	* gcc.target/riscv/rvv/autovec/partial/multiple_rgroup-1.h: New
	test.
	* gcc.target/riscv/rvv/autovec/partial/multiple_rgroup-2.c: New
	test.
	* gcc.target/riscv/rvv/autovec/partial/multiple_rgroup-2.h: New
	test.
	* gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-1.c:
	New test.
	* gcc.target/riscv/rvv/autovec/partial/multiple_rgroup_run-2.c:
	New test.