GlobalISel: don't emit degenerate G_INSERT instructions.

Before, we were producing G_INSERT instructions that were actually closer to a
cast or even a COPY when both input and output sizes are the same. This doesn't
really make sense and means that everything interpreting a G_INSERT also has to
handle all these kinds of casts.

So now we detect these degenerate cases and emit real casts instead.

llvm-svn: 297051