cmd/compile: don't rely on loop info when there are irreducible loops
Loop information is sketchy when there are irreducible loops.
Sometimes blocks inside 2 loops can be recorded as only being part of
the outer loop. That causes tighten to move values that want to move
into such a block to move out of the loop altogether, breaking the
invariant that operations have to be scheduled after their args.
Fixes #75569
Change-Id: Idd80e6d2268094b8ae6387563081fdc1e211856a
Reviewed-on: https://go-review.googlesource.com/c/go/+/706355 Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@google.com>