[RISCV] Pass vector argument by stack correctly.

We've a argument lowering logic to prevent floating-point value pass
passed with bit-conversion, but that rule should not applied to vector
arguments.

---

How to pass argument to `foo`:

```
tail call void @foo(i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0,
                    <vscale x 16 x float> zeroinitializer,
                    <vscale x 16 x float> zeroinitializer,
                    <vscale x 16 x float> zeroinitializer)
```

`foo` take 13 arguments, first 8 argument pass in GPR, and next 2 LMUL 8 vector
arguments passed in v8-v23, and now we run out of argument register for GPR and
vector register, so we must pass last LMUL 8 vector argument by stack.

Which means we should reserve `vlenb * 8` byte for stack for the last
vector argument.

Reviewed By: craig.topper, asb

Differential Revision: https://reviews.llvm.org/D145938