Simplifier panics ("Tick in rule") after SpecConstr and CSE due to `postInlineUnconditionally` inside RULE LHS
In !3187 I rather unexpectedly hit a compiler panic. To reproduce, build a validate-x86_64-linux-deb9-dwarf job (source notes `-g` and `-O2` for SpecConstr are essential) of !3187. Then you'll eventually get
```
ghc-stage1: panic! (the 'impossible' happened)
(GHC version 8.11.0.20200508:
Tick in rule
src<libraries/Cabal/Cabal/Distribution/Simple/GHC/Internal.hs:456:30-58>
src<libraries/Cabal/Cabal/Distribution/Simple/Flag.hs:95:1-13>
ds60_sskS
Call stack:
CallStack (from HasCallStack):
callStackDoc, called at compiler/GHC/Utils/Outputable.hs:1218:37 in ghc:GHC.Utils.Outputable
pprPanic, called at compiler/GHC/Core/Rules.hs:723:5 in ghc:GHC.Core.Rules
Please report this as a GHC bug: https://www.haskell.org/ghc/reportabug
```
while compiling libraries/Cabal/Cabal/Distribution/Simple/GHC.hs. I couldn't come up with a smaller reproducer yet. But I managed to pin-point the faulty behavior to a flaw in a rather [painful debugging process](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/3187#note_270974) to `postInlineUnconditionally`/occurrence analysis.
TLDR; As far as I can tell we need something like the `occ_rules_only` taint from `weakLoopBreaker` for `OccMany`/basically all other `OccInfo` fields to keep `postInlineUnconditionally` from triggering when a varaible occurs in an LHS of a RULE.
issue