Test stackmap support for i128

This diff adds tests that check the currently-working stackmap cases for i128.
This will help ensure no regressions are later introduced by D125680 (when
ready).

Note that i128 stackmap support is currently incomplete, so we cant test all
i128 functionality:

    i128 constants >= 2^{63} crash LLVM
    non-constant i128s crash LLVM

So this change tests only constant i128 operands of value < 2^{63}.

A couple of incorrect comments are also fixed.