[RISCV][ISel] Combine scalable vector add/sub/mul with zero/sign extension (#72340)

This PR mainly aims at resolving the below missed-optimization case,
while it could also be considered as an extension of the previous patch
https://reviews.llvm.org/D133739?id=

## Missed-Optimization Case
Compiler Explorer: https://godbolt.org/z/GzWzP7Pfh
### Source Code: 
```
define <vscale x 2 x i16> @multiple_users(ptr  %x, ptr  %y, ptr %z) {
  %a = load <vscale x 2 x i8>, ptr %x
  %b = load <vscale x 2 x i8>, ptr %y
  %b2 = load <vscale x 2 x i8>, ptr %z
  %c = sext <vscale x 2 x i8> %a to <vscale x 2 x i16>
  %d = sext <vscale x 2 x i8> %b to <vscale x 2 x i16>
  %d2 = sext <vscale x 2 x i8> %b2 to <vscale x 2 x i16>
  %e = mul <vscale x 2 x i16> %c, %d
  %f = add <vscale x 2 x i16> %c, %d2
  %g = sub <vscale x 2 x i16> %c, %d2
  %h = or <vscale x 2 x i16> %e, %f
  %i = or <vscale x 2 x i16> %h, %g
  ret <vscale x 2 x i16> %i
}
```
### Before This Patch
```
# %bb.0:
        vsetvli a3, zero, e16, mf2, ta, ma
        vle8.v  v8, (a0)
        vle8.v  v9, (a1)
        vle8.v  v10, (a2)
        svf2       v11, v8
        vsext.vf2       v8, v9
        vsext.vf2       v9, v10
        vmul.vv v8, v11, v8
        vadd.vv v10, v11, v9
        vsub.vv v9, v11, v9
        vor.vv  v8, v8, v10
        vor.vv  v8, v8, v9
        ret
```
###  After This Patch 
```
# %bb.0:
	vsetvli	a3, zero, e8, mf4, ta, ma
	vle8.v	v8, (a0)
	vle8.v	v9, (a1)
	vle8.v	v10, (a2)
	vwmul.vv	v11, v8, v9
	vwadd.vv	v9, v8, v10
	vwsub.vv	v12, v8, v10
	vsetvli	zero, zero, e16, mf2, ta, ma
	vor.vv	v8, v11, v9
	vor.vv	v8, v8, v12
	ret
```
We can see Add/Sub/Mul are combined with the Sign Extension.

## Relation to the Patch D133739
The patch D133739 introduced an optimization for folding `ADD_VL`/
`SUB_VL` / `MUL_V` with `VSEXT_VL` / `VZEXT_VL`. However, the patch did
not consider the case of non-fixed length vector case, thus this PR
could also be considered as an extension for the D133739.

Furthermore, in the current `SelectionDAG`, we represent scalable vector
add (or any binary operator) as a normal `ADD` operation. It might be
better to use an Opcode like `ADD_VL`, which needs further conversation
and decision.