[RISCV] Lower VLEFF/VLSEGFF SDNodes to MachineInstrs with VL outputs.

The patch is a replacement of D125199. PseudoReadVL with vtype has worry for
computing same vtypes of VLEFF/VLSEGFF in two different places, DAGToDAG and
InsertVSETVLI. VLEFF/VLSEGFF MI with VL output still could provide the vtype of
VLEFF/VLSEGFF to the users of its VL.

The patch names the new pseudo as original VLEFF/VLSEGFF name suffixed "_VL" and
expand them in RISCVInsertVSETVLI pass.

This patch also reverts commit 4537aae0,
"[RISCV] Make PseudoReadVL have the vtypes of the corresponding VLEFF/VLSEGFF.".

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D126794