IMPORTANT: Post-regalloc scheduling and JIT

Hi all,

I have turned on post- register allocation scheduling for x86 (Anton is investigating a problem on Linux, but it's *so far* working well on Darin). It's a very expensive codegen pass that can easily increase the codegen time by 10% or more. For this reason, I've added a new code generation optimization level CodeGenOpt::Less. If you are using the LLVM x86 JIT, you *should* initialize the with this optimization level to avoid running the pass (see LLVMTargetMachine::addPassesToEmitMachineCode).