[RISC-V Zbkb] gpr <=> vector roundtrip for bitreverse.vXi8 should become `brev8`

[Clang godbolt](https://llvm.godbo.lt/#g:!((g:!((g:!((h:codeEditor,i:(filename:'1',fontScale:14,fontUsePx:'0',j:1,lang:llvm,selection:(endColumn:52,endLineNumber:9,positionColumn:52,positionLineNumber:9,selectionStartColumn:52,selectionStartLineNumber:9,startColumn:52,startLineNumber:9),source:'define+i64+@foo(i64+%250)+unnamed_addr+align+4+%7B%0AEntry:%0A++%251+%3D+bitcast+i64+%250+to+%3C8+x+i8%3E%0A++%252+%3D+tail+call+%3C8+x+i8%3E+@llvm.bitreverse.v8i8(%3C8+x+i8%3E+%251)%0A++%253+%3D+bitcast+%3C8+x+i8%3E+%252+to+i64%0A++ret+i64+%253%0A%7D%0A%0Adeclare+%3C8+x+i8%3E+@llvm.bitreverse.v8i8(%3C8+x+i8%3E)+%231'),l:'5',n:'0',o:'LLVM+IR+source+%231',t:'0')),k:43.33886124930901,l:'4',n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:irclangtrunk,filters:(b:'0',binary:'1',binaryObject:'1',commentOnly:'0',debugCalls:'1',demangle:'0',directives:'0',execute:'1',intel:'0',libraryCode:'0',trim:'1',verboseDemangling:'0'),flagsViewOpen:'1',fontScale:14,fontUsePx:'0',j:1,lang:llvm,libs:!(),options:'-O3+-target+riscv64-linux+-mcpu%3Dxiangshan-kunminghu',overrides:!(),selection:(endColumn:1,endLineNumber:1,positionColumn:1,positionLineNumber:1,selectionStartColumn:1,selectionStartLineNumber:1,startColumn:1,startLineNumber:1),source:1),l:'5',n:'0',o:'+clang+(trunk)+(Editor+%231)',t:'0')),k:56.66113875069099,l:'4',m:100,n:'0',o:'',s:0,t:'0')),l:'2',n:'0',o:'',t:'0')),version:4)
```llvm
define i64 @foo(i64 %0) unnamed_addr align 4 {
Entry:
%1 = bitcast i64 %0 to <8 x i8>
%2 = tail call <8 x i8> @llvm.bitreverse.v8i8(<8 x i8> %1)
%3 = bitcast <8 x i8> %2 to i64
ret i64 %3
}

declare <8 x i8> @llvm.bitreverse.v8i8(<8 x i8>) #1
```

Currently:

```asm
foo:
vsetivli zero, 1, e64, m1, ta, ma
vmv.s.x v8, a0
vsetivli zero, 8, e8, mf2, ta, ma
vbrev.v v8, v8
vsetvli zero, zero, e64, m4, ta, ma
vmv.x.s a0, v8
ret
```

Could be:
```asm
foo:
brev8 a0, a0
ret
```