[GHC] #14375: Implement with# primop

[GHC]

#14375: Implement with# primop
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):  ​Phab:D4110    |         Wiki Page:
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 In Trac #14346 we proposed the new `with#` primop
 {{{
 with# :: a -> (State# s -> (# State# s, b #)) -> State# s -> (# State# s,
 b #)
 }}}
 This ticket is to track progress.

 See ​Phab:D4110.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14375>
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