[GHC] #15864: Fix Int8#, Int16#, Word8#, Word16# tests under LLVM code generator

[GHC]

#15864: Fix Int8#, Int16#, Word8#, Word16# tests under LLVM code generator
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Compiler       |           Version:  8.6.1
  (LLVM)                             |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Unfortunately it looks like LLVM doesn't like that we use `bitcast` to
 narrow integers. E.g. `CmpWord8` currently fails with:
 {{{
 =====> CmpWord8(llvm) 18 of 23 [0, 6, 0]
 cd "primops/should_run/CmpWord8.run" &&  "/mnt/work/ghc/ghc-
 compare-1/inplace/test   spaces/ghc-stage2" -o CmpWord8 CmpWord8.hs
 -dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn-missed-
 specialisations -fshow-warning-groups -fdiagnostics-color=never -fno-
 diagnostics-show-caret -Werror=compat -dno-debug-output  -fllvm
 Compile failed (exit code 1) errors were:
 [1 of 1] Compiling Main             ( CmpInt8.hs, CmpInt8.o )
 opt: /run/user/1000/ghc32287_0/ghc_2.ll:14549:20: error: invalid cast
 opcode for cast from 'i64' to 'i8'
   %ln6uE = bitcast i64 %ln6uD to i8
                    ^
 `opt' failed in phase `LLVM Optimiser'. (Exit code: 1)
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15864>
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