Auto merge of #37298 - nnethercote:faster-deflate, r=alexcrichton

Use a faster `deflate` setting

In #37086 we have considered various ideas for reducing the cost of LLVM bytecode compression. This PR implements the simplest of these: use a faster `deflate` setting. It's very simple and reduces the compression time by almost half while increasing the size of the resulting rlibs by only about 2%.

I looked at using zstd, which might be able to halve the compression time again. But integrating zstd is beyond my Rust FFI integration abilities at the moment -- it consists of a few dozen C files, has a non-trivial build system, etc. I decided it was worth getting a big chunk of the possible improvement with minimum effort.

The following table shows the before and after percentages of instructions executed during compression while doing debug builds of some of the rustc-benchmarks with a stage1 compiler.
```
html5ever-2016-08-25      1.4% -> 0.7%
hyper.0.5.0               3.8% -> 2.4%
inflate-0.1.0             1.0% -> 0.5%
piston-image-0.10.3       2.9% -> 1.8%
regex.0.1.30              3.4% -> 2.1%
rust-encoding-0.3.0       4.8% -> 2.9%
syntex-0.42.2             2.9% -> 1.8%
syntex-0.42.2-incr-clean 14.2% -> 8.9%
```
The omitted ones spend 0% of their time in decompression.

And here are actual timings:
```
futures-rs-test  4.110s vs  4.102s --> 1.002x faster (variance: 1.017x, 1.004x)
helloworld       0.223s vs  0.226s --> 0.986x faster (variance: 1.012x, 1.022x)
html5ever-2016-  4.218s vs  4.186s --> 1.008x faster (variance: 1.008x, 1.010x)
hyper.0.5.0      4.746s vs  4.661s --> 1.018x faster (variance: 1.002x, 1.016x)
inflate-0.1.0    4.194s vs  4.143s --> 1.012x faster (variance: 1.007x, 1.006x)
issue-32062-equ  0.317s vs  0.316s --> 1.001x faster (variance: 1.013x, 1.005x)
issue-32278-big  1.811s vs  1.825s --> 0.992x faster (variance: 1.014x, 1.006x)
jld-day15-parse  1.412s vs  1.412s --> 1.001x faster (variance: 1.019x, 1.008x)
piston-image-0. 11.058s vs 10.977s --> 1.007x faster (variance: 1.008x, 1.039x)
reddit-stress    2.331s vs  2.342s --> 0.995x faster (variance: 1.019x, 1.006x)
regex.0.1.30     2.294s vs  2.276s --> 1.008x faster (variance: 1.007x, 1.007x)
rust-encoding-0  1.963s vs  1.924s --> 1.020x faster (variance: 1.009x, 1.006x)
syntex-0.42.2   29.667s vs 29.391s --> 1.009x faster (variance: 1.002x, 1.023x)
syntex-0.42.2-i 15.257s vs 14.148s --> 1.078x faster (variance: 1.018x, 1.008x)
```

r? @alexcrichton

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