cmd/compile: fuzzing triggers various crashes or internal compiler errors on tip

What version of Go are you using (go version)?

Does this issue reproduce with the latest release?

Yes, tip.

What operating system and processor architecture are you using (go env)?

What did you do?

Fuzz cmd/compile using dvyukov/go-fuzz. (This is partially a follow-up to #39634, but there I was just fuzzing the type checker, whereas this was fuzzing the full cmd/compile).

Most of the fuzzing happened on an earlier commit, but roughly half of the original crashers that were initially found no longer reproduce on tip, or seem to be tracked elsewhere (e.g., https://github.com/golang/go/issues/47631#issuecomment-945007646).

What did you see?

Here are 4 crashers that still reproduce on latest tip with go tool compile:

Crash 1: goroutine stack exceeds 1000000000-byte limit

In types2.(*unifier).nifyEq (via nifyEq -> nify -> nifyEq -> nify -> ...)

package n

func O[T any](l, t func() T, f T) { O(t, func() g {}, func() {}) }

Crash 2: internal compiler error: label missing tag

In escape.(*escape).stmt:

package a

func A() {
_:
_:
}

Crash 3: internal compiler error: bvset: index 6 is out of bounds with length 6

In bitvec.BitVec.Set (via typebits.Set and liveness.WriteFuncMap):

package i

type w []w

func (w) a(_ [2 << 40]w)

Crash 4: internal compiler error: bad type

In types.typePos (via types.CalcSize):

package p

type F [][8][2][880][80000][80000]string

(FYI @mdempsky -- this is a belated follow-up from our conversation from a while ago).