PATCH: do ?: lowering to selects less wrong

[Nick Lewycky]

When lowering "cond ? X : Y" we do some safety checks to see whether we can
instead emit both X and Y and use a llvm select instruction to choose
between them. This code is insufficiently safe, and introducing loads into
a program that didn't load is a bad idea. For example, it could be TLS. It
could be a non-volatile auto in another function that isn't the current
function (think lambdas).

Don't do this here. LLVM knows how to do this properly.

Patch attached, please review!

Nick
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