ia32: Remove the last user of 'transform_upconv()'.
ia32: Remove the last user of 'transform_upconv()'.

Switch lowering always gives an unsigned selector, so 'transform_zext()' is sufficient.

MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,libfirm/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm