The Construct bytecode is always passed a valid feedback slot (just like
the Call bytecode), so no need to check for invalid feedback slot anymore.
Also perform the call count increment initially for both bytecodes instead
of delaying it, which decreases live range for the feedback vector and slot
registers.

R=mythria@chromium.org, rmcilroy@chromium.org
BUG=v8:4280

Review-Url: https://codereview.chromium.org/2955063002
Cr-Commit-Position: refs/heads/master@{#46232}