CodeReader visitor dispatch now uses OpcodeInfo.

CodeReader visitor dispatch now uses OpcodeInfo.

This gets rid of the per-opcode Instruction array and just does a
simple switch. If it turns out that the switch has poor performance,
we can push the visitor abstraction down into IndexType.

The big change to OpcodeInfo is to make every Info instance always
have a non-null IndexType, exactly so it could be reliably switch()ed
on without having to do an explicit null check.

Change-Id: Iee8289f8766ce41c5043eec7212b2133ec71e682