[PATCH] CodeGen: Don't iterate over operands after we've erased an MI

[Tim Northover via llvm-commits]

> This fixes a use-after-free introduced 3 years ago, in r182872 ;)

Oops.

> This seemed like the most NFC way to fix this, but I'm not terribly
> familiar with the RegisterCoalescer so let me know if there's a better
> way.

I'm not sure I'm very familiar either any more, but this looks like a reasonable change to me.

Tim.