cmd/compile: illegal combination ADD	$8, ZR, R2 ABCON0 ZREG NONE REG NONE, 4 7 on arm64

```
$ gotip version
go version devel go1.25-1647896aa2 Fri Apr 4 23:25:15 2025 -0700 linux/amd64
```
```
package p

func F(a, b map[float32]int) int {
	var st *struct {
		n int
		f float32
	}
	return a[0] + b[st.f]
}
```
```
$ GOARCH=arm64 gotip build crash.go 

./crash.go:8:2: illegal combination: 00068 (/home/alberto/crash.go:8)	ADD	$8, ZR, R2 ABCON0 ZREG NONE REG NONE, 4 7
```

cc @randall77 