[PATCH] IR: Check inalloca call argument originating from parameter

[Reid Kleckner]

So, we don't generally fail verification because somebody gave us well-formed but nonsensical IR.  I kind of compromised on the last verifier change because I think it will save us lots of time, and we can revert it if somebody objects.  Consider the FAQ about calling convention mismatches:

http://llvm.org/docs/FAQ.html#why-does-instcombine-simplifycfg-turn-a-call-to-a-function-with-a-mismatched-calling-convention-into-unreachable-why-not-make-the-verifier-reject-it

It's not clear to me that this new check will find lots of optimizer bugs for us, but I could be proven wrong, and yes, it might take more time to figure it out without a verifier check.  Anyway, I think we need a second opinion to keep going in this direction, so I added Nick.

If we do go this direction, we can strengthen this check to fail if the call site isn't musttail, because the argument memory won't match up unless tail call succeeds.

http://reviews.llvm.org/D3593