Minor cleanup of foldRegs{Used,Defd}

This makes the two functions strict in the accumulator - it seems that
there are only two users of those functions: `CmmLive` and `CmmSink`
and in both cases the strict fold fits better.

The commit also removes a few unused functions (`filterRegsUsed`),
instances (for `Maybe` and `RegSet`) and gets rid of unnecessary
inculde of `HsVersions.h`.

The performance effect of avoiding unnecessary thunks is mostly
negligible, although we do allocate a tiny bit less (nofib's section
on compile allocations):
```
-1 s.d.                -----            -0.2%
+1 s.d.                -----            -0.1%
Average                -----            -0.2%
```

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

Test Plan: validate

Reviewers: simonmar, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2723

0 files changed, 0 insertions, 0 deletions