[PATCH] x86 NaCl: Emit LAHF/SAHF instead of PUSHF/POPF

[JF Bastien]

Alright, here's what's left to figure out:

- Is liveness maintenance correct?
- Should I handle individual flags more precisely? That means I could emit as little as a single SETcc, or just LAHF/SAHF and omit SETO.
- Should I make the NaCl path the only one (e.g. no PUSHF/POPF)?
- Refactor the code, if pulling out a function makes sense.


http://reviews.llvm.org/D6629

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