Report registers as dead in GCInfo before the RhpPInvoke helper.

[sandreenko]

CORINFO_HELP_JIT_PINVOKE_BEGIN makes transition to preemptive mode before a P/Invoke, so it requires GC pointer state to be cleared before the helper.

I checked other occurrences of IsUnmanaged() and looks like they already consider or do not need to consider this helper case.

If opts.ShouldUsePInvokeHelpers() is enabled, than the state before the P/Invoke must be clean, so we can rewrite it as:

if (opts.ShouldUsePInvokeHelpers())
{
    if (call->gtCallMethHnd == eeFindHelper(CORINFO_HELP_JIT_PINVOKE_BEGIN))
    {
        return true;
    }
    else if (call->IsUnmanaged())
    {
       assert that the state has been cleared already.
       return false;
    }
}
else if (call->IsUnmanaged())
{
    return true;
}

but it doesn't harm to return true for unmanaged calls always.

Fix dotnet/corert#4676 .

Checked that it fixes the original test.

[sandreenko]

PTAL @jkotas , @dotnet/jit-contrib

[jkotas]

Thanks!

[briansull]

Looks Good