[llvm] [RISCV] Separate doLocalPostpass into new pass and move to post vector regalloc (PR #88295)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

@lukel97 

After this patch, additional `vmv.v.v` instructions will be generated:
Godbolt: https://godbolt.org/z/z99oMq663
```
define <4 x i64> @func(<4 x i64> %0, <4 x i64> %1) nounwind {
entry:
  %2 = mul <4 x i64> %0, <i64 -64, i64 -64, i64 -64, i64 -64>
  %3 = sub <4 x i64> <i64 -16, i64 -16, i64 -16, i64 -16>, %1
  %4 = icmp eq <4 x i64> %0, <i64 1, i64 1, i64 1, i64 1>
  %5 = select <4 x i1> %4, <4 x i64> %3, <4 x i64> %2
  ret <4 x i64> %5
}
```

Before:
```
func:
 	vsetivli	zero, 4, e64, m2, ta, mu
 	vsll.vi	v12, v8, 6
 	vmseq.vi	v0, v8, 1
 	vrsub.vi	v8, v12, 0
 	vrsub.vi	v8, v10, -16, v0.t
 	ret
```

After:
```
func:
        vsetivli        zero, 4, e64, m2, ta, mu
        vsll.vi v12, v8, 6
        vrsub.vi        v12, v12, 0
        vmseq.vi        v0, v8, 1
        vrsub.vi        v12, v10, -16, v0.t
        vmv.v.v v8, v12
        ret
```

See https://github.com/dtcxzyw/llvm-codegen-benchmark/commit/0b07efec1ade06addd3c5032d6c651b4c4123653 for more cases.



https://github.com/llvm/llvm-project/pull/88295