Fix TcType.anyRewritableTyVar

Authored by ningning on Oct 28 2018, 11:29 AM.

Description

Fix TcType.anyRewritableTyVar

Summary:
This patch fixes Trac #15805, where we found that
TcType.anyRewritableTyVar has one wrong case.

Besides the fix, it also:

  • removed some unnecessary ASSERT2(tcIsTcTyVar...) in TcType, as now we have tcIsTcTyVar = isTyVar.
  • fixed some comments

Test Plan: ./validate

Reviewers: goldfire, simonpj, bgamari

Reviewed By: simonpj

Subscribers: rwbarton, carter

GHC Trac Issues: Trac #15805

Differential Revision: https://phabricator.haskell.org/D5263