Before this change, the compiler could panic with the following message:
panic: 20 not an Int
That of course doesn't make much sense. But it apparently is expected
behavior, see https://github.com/golang/go/issues/43165 for details.
This commit fixes this issue by converting the constant to an integer if
needed.
Previously, the compiler used LLVM's shift instructions directly, which have UB whenever the shifts are large or negative.
This commit adds runtime checks for negative shifts, and handles oversized shifts.
