RISC-V: Fix ratio in vsetvl fuse rule [PR115703].

In PR115703 we fuse two vsetvls:

    Fuse curr info since prev info compatible with it:
      prev_info: VALID (insn 438, bb 2)
        Demand fields: demand_ge_sew demand_non_zero_avl
        SEW=32, VLMUL=m1, RATIO=32, MAX_SEW=64
        TAIL_POLICY=agnostic, MASK_POLICY=agnostic
        AVL=(reg:DI 0 zero)
        VL=(reg:DI 9 s1 [312])
      curr_info: VALID (insn 92, bb 20)
        Demand fields: demand_ratio_and_ge_sew demand_avl
        SEW=64, VLMUL=m1, RATIO=64, MAX_SEW=64
        TAIL_POLICY=agnostic, MASK_POLICY=agnostic
        AVL=(const_int 4 [0x4])
        VL=(nil)
      prev_info after fused: VALID (insn 438, bb 2)
        Demand fields: demand_ratio_and_ge_sew demand_avl
        SEW=64, VLMUL=mf2, RATIO=64, MAX_SEW=64
        TAIL_POLICY=agnostic, MASK_POLICY=agnostic
        AVL=(const_int 4 [0x4])
        VL=(nil).

The result is vsetvl zero, zero, e64, mf2, ta, ma.  The previous vsetvl
set vl = 4 but here we wrongly set it to vl = 2.  As all the following
vsetvls only ever change the ratio we never recover.

The issue is quite difficult to trigger because we can often
deduce the value of d at runtime.  Then very check for the value of
d will be optimized away.

The last known bad commit is r15-3458-g5326306e7d9d36.  With that commit
the output is wrong but -fno-schedule-insns makes it correct.  From the
next commit on the issue is latent.  I still added the PR's test as scan
and run check even if they don't trigger right now.  Not sure if the
run test will ever fail but well.  I verified that the
patch fixes the issue when applied on top of r15-3458-g5326306e7d9d36.

	PR target/115703

gcc/ChangeLog:

	* config/riscv/riscv-vsetvl.cc: Use max_sew for calculating the
	new LMUL.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr115703-run.c: New test.
	* gcc.target/riscv/rvv/autovec/pr115703.c: New test.