[RISCV] Teach VSETVLI insertion to merge the unused VSETVLI with the one need...

[RISCV] Teach VSETVLI insertion to merge the unused VSETVLI with the one need to be insert after it.

If a vsetvli instruction is not compatible with the next vector instruction,
and there is no other things that may update or use VL/VTYPE, we could merge
it with the next vsetvli instruction that should be insert for the vector
instruction.

This commit only merge VTYPE with the former vsetvli instruction which has
the same VL.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D106857