LoopUnroll: If we're doing partial unrolling, use the PartialThreshold to limit unrolling.

Otherwise we use the same threshold as for complete unrolling, which is
way too high. This made us unroll any loop smaller than 150 instructions
by 8 times, but only if someone specified -march=core2 or better,
which happens to be the default on darwin.

llvm-svn: 207940