patch: emit tail calls with zeroext/signext on return

[Nick Lewycky]

Currently clang can't emit a tail call on:

  bool g(); bool f() { return g(); }

because we lower the functions to have "zeroext i1" return. This patch
updates the safety check in codegen which currently forbids zeroext/signext
from ever participating in tail calls to instead check that the caller and
callee match.

Please review!

Nick
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