Implement first cut at optimizer step 2. Limitations:

Implement first cut at optimizer step 2.  Limitations:
- Does not detect cycles, so will infinite loop.
- Does not yet handle secondary expansion of bc offset bytecodes.
However, dwarf64_leb128 now generates essentially identical code to GAS
(except GAS generates relocs for in-file function refs, whereas yasm does not)
x86id tests also now work, but have not been checked yet (so no checkin of
new golden results yet).

svn path=/branches/new-optimizer/; revision=1578