See #15696 for more details. We now always enter dataToTag# argument (done in
generated Cmm, in StgCmmExpr). Any high-level optimisations on dataToTag#
applications are done by the simplifier. Looking at tag bits (instead of
reading the info table) for small types is left to another diff.

Incorrect test T14626 is removed. We no longer do this optimisation (see
comment:44, comment:45, comment:60).

Comments and notes about special cases around dataToTag# are removed. We no
longer have any special cases around it in Core.

Other changes related to evaluating primops (seq# and dataToTag#) will be
pursued in follow-up diffs.

Test Plan: Validates with three regression tests

Reviewers: simonpj, simonmar, hvr, bgamari, dfeuer

Reviewed By: simonmar

Subscribers: rwbarton, carter

GHC Trac Issues: #15696

Differential Revision: https://phabricator.haskell.org/D5201