mapreduce: avoid deadlock by forcing the accumulator type.

[maleadt]

Otherwise we may union-split across a shfl invocation, resulting in a deadlock.

Fixes #2595

[maleadt]

These segfaults are annoying; only seem to happen on CI...

Thread 6 "julia" received signal SIGSEGV, Segmentation fault.
[Switching to LWP 1179041]
ijl_process_events () at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/jl_uv.c:389
389	/cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/jl_uv.c: No such file or directory.
(gdb) bt
#0  ijl_process_events () at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/jl_uv.c:389
#1  0x00007ffff71d3f37 in ijl_task_get_next (trypoptask=<optimized out>, q=<optimized out>,
    checkempty=<optimized out>) at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/scheduler.c:610
#2  0x00007fffe25a9993 in julia_poptask_66344 () at task.jl:1012
#3  0x00007fffe386f313 in julia_wait_65850 () at task.jl:1021
#4  0x00007fffe30d4494 in julia_#wait#731_65869 () at condition.jl:130
#5  0x00007fff4cf399ae in ?? ()
#6  0x00007fff16d30010 in ?? ()
#7  0x389b91a0f883f464 in ?? ()
#8  0x00007fff31b83c28 in ?? ()
#9  0x00007fff16d30080 in ?? ()
#10 0x00007fff31b83cd0 in ?? ()
#11 0x00007fff31b83ad0 in ?? ()
#12 0x389b91a0f403f464 in ?? ()
#13 0x389b6b37b033f464 in ?? ()
#14 0x00007fff00000000 in ?? ()
#15 0x00007fff103c68c0 in ?? ()
#16 0x00007fff103afa70 in ?? ()
#17 0x0000000000000004 in ?? ()
#18 0x00007fff103c5c60 in ?? ()
#19 0x0000000000000008 in ?? ()
#20 0x0000000000000040 in ?? ()
#21 0x0000000000000000 in ?? ()

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.63%. Comparing base (4e9513b) to head (22a89f9).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2596      +/-   ##
==========================================
- Coverage   73.64%   73.63%   -0.02%     
==========================================
  Files         157      157              
  Lines       15204    15204              
==========================================
- Hits        11197    11195       -2     
- Misses       4007     4009       +2     

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