[LLVMdev] Let's -disable-post-ra for x86

[Jakob Stoklund Olesen]

We are currently running the post-ra scheduler for x86 with -O3. It is not really clear that it is worth the compile time.

X86 scheduling is a mystery that post-ra has not solved.

I ran the nightly test suite on x86_64 with LTO enabled and -O3 with and without -disable-post-ra. Code generation is 10% faster overall with -disable-post-ra, and the runtime impact varies from a 17% speedup to a 17 percent slowdown.

Below are the tests that had a more than 3% change in runtime.

I think we should disable post-ra scheduling for x86.

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