RISC-V: Refine LMUL computation for MASK_LEN_LOAD/MASK_LEN_STORE IFN

Notice a case has "Maximum lmul = 16" which is incorrect.
Correct LMUL estimation for MASK_LEN_LOAD/MASK_LEN_STORE.

Committed.

gcc/ChangeLog:

	* config/riscv/riscv-vector-costs.cc (variable_vectorized_p): New function.
	(compute_nregs_for_mode): Refine LMUL.
	(max_number_of_live_regs): Ditto.
	(compute_estimated_lmul): Ditto.
	(has_unexpected_spills_p): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-11.c: New test.