an optomization to remove redundant cast operations.

1. Adds an optimization to remove redundant ToBoolean and ToName operations.
2. Adds implementation and tests for cast operatorts to bytecode graph builder.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1468003002

Cr-Commit-Position: refs/heads/master@{#32408}