This refactors how the BytecodeArrayIterator is passed to visitation
methods on the BytecodeGraphBuilder. We no longer pass it explicitly,
but use the field accessor instead. Note that const-ness is still
preserved and visitation methods are still not able to mutate the
iterator. The main goal of this refactoring is increased readability.

R=rmcilroy@chromium.org

Review URL: https://codereview.chromium.org/1642893004

Cr-Commit-Position: refs/heads/master@{#33607}