[GHC] #15364: Replace the atomicModifyMutVar# primop

[GHC]

#15364: Replace the atomicModifyMutVar# primop
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Runtime        |           Version:  8.5
  System                             |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):  Phab:D4884     |         Wiki Page:
-------------------------------------+-------------------------------------
 This was [https://github.com/ghc-proposals/ghc-
 proposals/blob/master/proposals/0027-atomicModifyMutVar.rst accepted
 proposal #27]. Specifically:

 1. Replace `atomicModifyMutVar#` with `atomicModifyMutVar2#`.
 2. Add `atomicModifyMutVar_#` and `atomicSwapMutVar#`.
 3. Add a function imitating `atomicModifyMutVar#` to `GHC.Exts` for
 backwards compatibility.
 4. Use the new primops where appropriate throughout `base`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15364>
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