RISC-V: Fix RVV_VLMAX

RISC-V: Fix RVV_VLMAX

This patch fixes memory hog found in SPEC2017 wrf benchmark which caused by
RVV_VLMAX since RVV_VLMAX generate brand new rtx by gen_rtx_REG (Pmode, X0_REGNUM)
every time we call RVV_VLMAX, that is, we are always generating garbage and redundant
(reg:DI 0 zero) rtx.

After this patch fix, the memory hog is gone.

Time variable                                   usr           sys          wall           GGC
 machine dep reorg                  :   1.99 (  9%)   0.35 ( 56%)   2.33 ( 10%)   939M ( 80%) [Before this patch]
 machine dep reorg                  :   1.71 (  6%)   0.16 ( 27%)   3.77 (  6%)   659k (  0%) [After this patch]

Time variable                                   usr           sys          wall           GGC
 machine dep reorg                  :  75.93 ( 18%)  14.23 ( 88%)  90.15 ( 21%) 33383M ( 95%) [Before this patch]
 machine dep reorg                  :  56.00 ( 14%)   7.92 ( 77%)  63.93 ( 15%)  4361k (  0%) [After this patch]

Test is running. Ok for trunk if I passed the test with no regresion ?

	PR target/113495

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (RVV_VLMAX): Change to regno_reg_rtx[X0_REGNUM].
	(RVV_VUNDEF): Ditto.
	* config/riscv/riscv-vsetvl.cc: Add timevar.