Adds support for the LdaGlobal and StaGlobal bytecodes to the
BytecodeGraphBuilder. Also fixes a bug in the context node's parameter
index and start node inputs.

Landed on behalf of rmcilroy.

TBR=bmeuer@chromium.org,mythria@chromium.org
BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1449373002

Cr-Commit-Position: refs/heads/master@{#32049}