Greatly speed up nativeCodeGen/seqBlocks

Authored by nomeata.

Description

Greatly speed up nativeCodeGen/seqBlocks

When working on Trac #10397, I noticed that "reorder" in
nativeCodeGen/seqBlocks took more than 60% of the time. With this
refactoring, it does not even show up in the profile any more. This
fixes Trac #10422.

Differential Revision: https://phabricator.haskell.org/D893