core/types: optimize modernSigner.Equal

[cuiweixie]

goos: darwin
goarch: arm64
pkg: github.com/ethereum/go-ethereum/core/types
cpu: Apple M1 Pro
                       │   old.txt    │               new.txt               │
                       │    sec/op    │   sec/op     vs base                │
_modernSigner_Equal-10   101.65n ± 1%   10.29n ± 0%  -89.88% (p=0.000 n=20)

                       │  old.txt   │            new.txt             │
                       │    B/op    │    B/op     vs base            │
_modernSigner_Equal-10   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=20) ¹
¹ all samples are equal

                       │  old.txt   │            new.txt             │
                       │ allocs/op  │ allocs/op   vs base            │
_modernSigner_Equal-10   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=20) ¹
¹ all samples are equal