compiler: optimize 0,1,2-case select statement

compiler: optimize 0,1,2-case select statement

    For a select statement with zero-, one-, or two-case with a
    default case, we can generate simpler code instead of calling the
    generic selectgo. A zero-case select is just blocking the
    execution. A one-case select is mostly just executing the case. A
    two-case select with a default case is a non-blocking send or
    receive. We add these special cases for lowering a select
    statement.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/184998

From-SVN: r273034