avoid setting `tail` for `@panic`

Currently, slices are passed via reference, even though it would be
better to pass the ptr and len as separate arguments (#561). This means
that any function call with a slice parameter cannot be a tail call,
because according to LLVM spec:

> Both [tail,musttail] markers imply that the callee does not access
> allocas from the caller

There was one other place we were setting `tail` and I made that
conditional on whether or not the argument referenced allocas in the
caller.

This was causing undefined behavior in the compiler when it hit asserts,
causing it to print garbage memory to the terminal. See #3262 for
example.

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