Introduce a proper CodeStubAssembler::BranchIfToBooleanIsTrue helper
method, that branches to if_true/if_false labels depending on whether
the value that is passed would yield true or false when fed to
ToBoolean. Use this helper to implement the bytecode handlers w/o having
to materialize the temporary booleans and essentially branching twice.
The CodeStubAssembler::BranchIfToBooleanIsTrue helper favors the most
likely case of a Boolean constant now.

Also migrate the ToBooleanStub to a ToBoolean TurboFan builtin, that
also uses the helper method under the hood.

Remove the now obsolete Oddball::to_boolean field.

R=hpayer@chromium.org, rmcilroy@chromium.org, yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2151163002
Cr-Commit-Position: refs/heads/master@{#37849}