[RISCV] Teach vsetvli insertion to not insert redundant vsetvli right after VLEFF...

[RISCV] Teach vsetvli insertion to not insert redundant vsetvli right after VLEFF/VLSEGFF.

VSETVLIInfos right after VLEFF/VLSEGFF are currently unknown since they modify
VL. Unknown VSETVLIInfos make next vector operations needed to be inserted
VSET(I)VLI. Actually the next vector operation of VLEFF/VLSEGFF may not need to
be inserted VSET(I)VLI if it uses same VTYPE and the resulted vl of
VLEFF/VLSEGFF.

Take the below C code as an example,

  vint8m4_t vec_src1 = vle8ff_v_i8m4(str1, &new_vl, vl);
  vbool2_t mask1 = vmseq_vx_i8m4_b2(vec_src1, 0, new_vl);
  vsetvli insertion adds a redundant vsetvli for that,

Assembly result:
  vsetvli a2,a2,e8,m4,ta,mu
  vle8ff.v v28,(a0)
  csrr a3,vl ; redundant
  vsetvli zero,a3,e8,m4,ta,mu ; redundant
  vmseq.vi v25,v28,0

After D126794, VLEFF/VLSEGFF has a define having value of VL. The patch consider
there is a ghost vsetvli right after VLEFF/VLSEGFF. The ghost VSET(I)LIs use the
vl output of the VLEFF/VLSEGFF as its AVL and same VTYPE of the VLEFF/VLSEGFF.
The ghost vsetvli must be redundant, and we could use it to get the VSETVLIInfo
right after VLEFF/VLSEGFF.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D127576