Add @likely/@unlikely annotations for blocks

[ChrisDodd]

These will generally be used by backends and simply passed through the frontend/midend, so not much is needed here.

Perhaps some frontend passes (in particular, DoSimplifyControlFlow in frontends/p4/simplify.cpp) could be extended to understand these -- currently, any annotations on a block will inhibit combining the blocks, whereas @likely/@unlikely on a block should not inhibit combining it with a block with no annotation (the annotation should be on the resulting combined block).

See discussion in p4lang/p4-spec#1308

[vlstill]

Perhaps some frontend passes (in particular, DoSimplifyControlFlow in frontends/p4/simplify.cpp) could be extended to understand these -- currently, any annotations on a block will inhibit combining the blocks, whereas @likely/@unlikely on a block should not inhibit combining it with a block with no annotation (the annotation should be on the resulting combined block).

Sounds reasonable. Is there a risk that the annotation will currently inhibit some optimization? That would be very unfortunate.

However, if we start merging blocks where the annotations don't match exactly, could we end up with both @likely and @unlikely on a single blocks? What should be done then? Imagine code like this:

if (some_runtime_condition) @likely {
    if (some_condition_compiler_proves_true) @unlikely {
        code;
    }
}

obviously, this example is a bit pathological as the code claims the branch is unlikely, but the compiler proves it can be taken always.