StgLint overhaul

- Remove all type checks
- Check two STG invariants (no unboxed let bindings, variables defined
  before used) and post-unarisation invariants.

See the module header and #14787.

This version validates with `-dstg-lint` added to `GhcStage2HcOpts` and
`GhcLibHcOpts` and `EXTRA_HC_OPTS`.

Unarise changes:

- `unariseConArgBinder` and `unariseFunArgBinder` functions were almost
  the same; only difference was when unarising fun args we keep void
  args while in con args we drop them. A new function `unariseArgBinder`
  added with a `Bool` argument for whether we're unarising a con arg.
  `unariseConArgBinder` and `unariseFunArgBinder` are now defined as

      unariseConArgBinder = unarsieArgBinder True  -- data con
      unariseFunArgBinder = unariseArgBinder False -- not data con

- A bug in `unariseConArgBinder` and `unariseFunArgBinder` (which are
  just calls to `unariseArgBinder` now) that invalidated the
  post-unarise invariants when the argument has single type rep (i.e.
  `length (typePrimRep x) == 1`) fixed. This isn't a correctness issue
  (it's fine not to unarise if a variable is already represented as
  single value), but it triggers StgLint.

Test Plan:
- Pass testsuite with `-dstg-lint` [done]
- Boot stage2 (including libraries) with `-dstg-lint` [done]

Reviewers: simonpj, bgamari

Reviewed By: bgamari

Subscribers: duog, rwbarton, thomie, carter

GHC Trac Issues: #14787

Differential Revision: https://phabricator.haskell.org/D4404

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