This renames the {operand_stack} field to {register_file}, to refelct
how said field is used on all {JSGeneratorObject} instances by now. This
is a pure refactoring CL, not changes in semantics.

R=neis@chromium.org

Review-Url: https://codereview.chromium.org/2520913002
Cr-Commit-Position: refs/heads/master@{#41140}