compiler: use int type for len & cap in slice value

compiler: use int type for len & cap in slice value

    Slice value expression has backend type a struct of a pointer and
    two ints. Make sure the len and cap are converted to int when
    creating slice value expression.

    Reviewed-on: https://go-review.googlesource.com/c/156897

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@267745 138bc75d-0d04-0410-961f-82ee72b054a4