[RISCV] Use vmv.v.[v|i] if we know COPY is under the same vl and vtype.

[RISCV] Use vmv.v.[v|i] if we know COPY is under the same vl and vtype.

If we know the source operand of COPY is defined by a vector instruction
with tail agnostic and the same LMUL and there is no vsetvli between
COPY and the define instruction to change the vl and vtype, we could use
vmv.v.v or vmv.v.i to copy vector registers to get better performance than
the whole vector register move instructions.

If the source of COPY is from vmv.v.i, we could use vmv.v.i for the
COPY.

This patch only considers all these instructions within one basic block.

Case 1:
```
bb.0:
  ...
  VSETVLI          # The first VSETVLI before COPY and VOP.
  ...              # Use this VSETVLI to check LMUL and tail agnostic.
  ...
  vy = VOP va, vb  # Define vy.
  ...              # There is no vsetvli between VOP and COPY.
  vx = COPY vy
```

Case 2:
```
bb.0:
  ...
  VSETVLI          # The first VSETVLI before VOP.
  ...              # Use this VSETVLI to check LMUL and tail agnostic.
  ...
  vy = VOP va, vb  # Define vy.
  ...              # There is no vsetvli to change vl between VOP and COPY.
  ...
  VSETVLI          # The first VSETVLI before COPY.
  ...              # This VSETVLI does not change vl and vtype.
  ...
  vx = COPY vy
```

Co-Authored-by: Zakk Chen <zakk.chen@sifive.com>
Co-Authored-by: Kito Cheng <kito.cheng@sifive.com>
Differential Revision: https://reviews.llvm.org/D103510