Fix code generation making liveness pass unhappy

[elliottslaughter]

As of 668e2c8, we have enabled the LiveIRVariables LLVM pass which is a part of ongoing GC work for #2997.

Unfortunately, something in test/bench/task-perf-word-count.rs makes the LiveIRVariables pass unhappy. It seems that some LLVM optimization pass is producing an irreducible control flow graph, and LLVM's LoopSimplify pass isn't sophisticated enough to undo the damage.

As a temporary workaround, 43867bf xfails the two tests which break the liveness pass. But we should figure out what LLVM pass is causing the trouble and try to resolve that and un-xfail those tests.

[elliottslaughter]

Fixed in 9c9de0b, and workaround reverted in 416584d.