compiler: adjust code to avoid shadowing local variables

compiler: adjust code to avoid shadowing local variables

    Also add a couple of missing calls to free after mpz_get_str.

    This should make the code clean with respect to -Wshadow=local.

    Based on patch by Bernd Edlinger.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/198837

From-SVN: r276579