Adds support for short operands, starting with kIdx16. Introduces
BytecodeTraits to enable compile time determination of various traits for a
bytecode, such as size, operands, etc. Reworks BytecodeIterator,
BytecodeArrayBuilder and Bytecodes::Decode to support 16 bit operands. Adds
support to Interpreter to load 16 bit operands.

Also fixes a bug with ToBoolean where it wouldn't get emitted at the start
of a block, and added a test.

BytecodeTraits template magic inspired by oth@chromium.org.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1370893002

Cr-Commit-Position: refs/heads/master@{#31058}