compiler: fix loopdepth tracking in array slicing expression in escape analysis

compiler: fix loopdepth tracking in array slicing expression in escape analysis

    In the gc compiler, for slicing an array, its AST has an implicit
    address operation node. There isn't such node in the gofrontend
    AST. During the escape analysis, we create a fake node to mimic
    the gc compiler's behavior. For the fake node, the loopdepth was
    not tracked correctly, causing miscompilation. Since this is an
    address operation, do the same thing as we do for the address
    operator.

    Fixes golang/go#36404.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/213643

From-SVN: r279984