compiler: annotate or fix fall through cases in switch statements

compiler: annotate or fix fall through cases in switch statements

    A new GCC warning, -Wimplicit-fallthrough, detected these three cases
    where a switch case falls through to another.  One of them looks like
    a bug, so I fixed it; the two other look like intended fall throughs
    so I just added a "fall through" comment to suppress the GCC warning.

    Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.

    Reviewed-on: https://go-review.googlesource.com/26695

From-SVN: r239332