Benchmarking some lld revisions

[Rafael Espíndola via llvm-commits]

Now that I have a workstation setup again for doing really stable
benchmarks I decided to try a quick experiment and benchmark linking
firefox with the last 1000 revisions or so.

I used the last 1000 revision, regardless of what they changed. That
way we can get an idea of the fluctuation to expect from measurement
bias when unrelated things change.

The results are attached. Data has 3 columns. The first one is the
revision, the second one is the time and the third is the percentage
std dev reported by perf over 10 runs.

The plot was created with gnuplot:

plot "data" using 1:2:($3 * $2 /100) title "time" with errorlines

It is interesting to see that the variation from commit to commit is
much bigger than the variation in a single commit, but the graph
allows us to find some significant commits. In this case, they were:

r303689: The test has --buildit ... --build-id=none, and now the
second one is used.

r303925: Uses CachedHashStringRef again for comdat signatures.

Cheers,
Rafael

P.S.: It would be truly awesome if someone could setup a bot that does
this over various tests.
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