[GHC] #14439: Remove redundant subtraction in (^) and stimes

[GHC]

#14439: Remove redundant subtraction in (^) and stimes
-------------------------------------+-------------------------------------
        Reporter:  Bodigrim          |                Owner:  (none)
            Type:  task              |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  libraries/base    |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4173
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"eb5a40cea6c64f5300c7697231cb0ede2c554388/ghc"
 eb5a40ce/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="eb5a40cea6c64f5300c7697231cb0ede2c554388"
 base: Remove redundant subtraction in (^) and stimes

 Subtraction `y - 1` is redundant. The value of y is guaranteed to be
 positive and odd, so
 ```
 (y - 1) `quot` 2` = `y `quot` 2
 ```

 Test Plan: validate

 Reviewers: hvr, bgamari

 Reviewed By: bgamari

 Subscribers: rwbarton, thomie

 GHC Trac Issues: #14439

 Differential Revision: https://phabricator.haskell.org/D4173
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14439#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler