Fix performance problem in LLVM JumpThreading

[Keno]

This patch is https://reviews.llvm.org/D42262 upstream. When we emit
unboxed unions, we tend to emit switch instructions on a small
integer which serves as a marker for which of types is currently active,
so we often emit things like:

block:
    ; Incoming from a union split branch
    %phi = i8 phi [1, %a], [2, %b]
    < Some other operation not on the union split>
    ; Union split again
    switch i8 %phi, label %boxed [
        i8 1, %abranch
        i8 2, %bbranch
    ]

In many situations the operation in the middle can get optimized away,
so we want to merge the two union split sections into one. LLVM's
jump threading pass can do this by keeping track of how control flow
behaves across a given basic block. Unfortunately, this optimization
wasn't taking effect. This is because InstCombine realized that the
range of possible values was rather small and turned the above into
something like:

   %trunc = truc i8 %phi to i2
   switch i2 %trunc, label %boxed [
       i2 1, %abranch
       i2 -2, %bbranch
   ]

which JumpThreading refused to look through (because of the i2 rather
than the i1). The included patch fixes this. On recent LLVM, we
additionally need https://reviews.llvm.org/D42260, for cases where
a union split branch happens to target a loop header. However,
LLVM 3.9.1 does not include the original commit that regressed that.

This fixes a number of the performance regressions seen in #25261.

[Keno]

@nanosoldier runbenchmarks(ALL, vs=":master")

[nanosoldier]

Something went wrong when running your job:

NanosoldierError: failed to run benchmarks against primary commit: failed process: Process(`sudo cset shield -e su nanosoldier -- -c ./benchscript.sh`, ProcessExited(1)) [1]

Logs and partial data can be found here
cc @ararslan

[Keno]

cc @ararslan LinAlg move broke nanosoldier

[ararslan]

Ah, so it did. Thanks for the heads up, I'll get on that right away.

[ararslan]

@nanosoldier runbenchmarks(ALL, vs=":master")

[nanosoldier]

Something went wrong when running your job:

NanosoldierError: failed to run benchmarks against primary commit: failed process: Process(`sudo cset shield -e su nanosoldier -- -c ./benchscript.sh`, ProcessExited(1)) [1]

Logs and partial data can be found here
cc @ararslan

[ararslan]

@nanosoldier runbenchmarks(ALL, vs=":master")

[nanosoldier]

Something went wrong when running your job:

NanosoldierError: failed to run benchmarks against primary commit: failed process: Process(`sudo cset shield -e su nanosoldier -- -c ./benchscript.sh`, ProcessExited(1)) [1]

Logs and partial data can be found here
cc @ararslan

[Keno]

Looks like the Serializer move also broke it.

[ararslan]

Thanks, working on that now.

[ararslan]

@nanosoldier runbenchmarks(ALL, vs=":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here. cc @ararslan