[llvm] [RISCV] Custom legalize f16/bf16 FNEG/FABS with Zfhmin/Zbfmin. (PR #106886)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw approved this pull request.

LG. BTW we have the same problem with f16/bf16 fcopysign.
```
; bin/llc -mtriple=riscv64 -mattr=+zfhmin test.ll -o -
define half @test(half %a, half %b) nounwind {
  %t = call half @llvm.copysign.f16(half %a, half %b)
  ret half %t
}
```
```
        addi    sp, sp, -16
        fsh     fa1, 8(sp)
        fsh     fa0, 0(sp)
        lbu     a0, 9(sp)
        lbu     a1, 1(sp)
        andi    a0, a0, 128
        andi    a1, a1, 127
        or      a0, a1, a0
        sb      a0, 1(sp)
        flh     fa0, 0(sp)
        addi    sp, sp, 16
        ret
```

https://github.com/llvm/llvm-project/pull/106886