Optimize OP_ADD instruction.
Optimize OP_SUBI.
Remove mrb_checkstack() as unused.
Make some tiny functions inlined. This is for supeed tuning.
Extract uvget/uvset to OP_GETUPVAR/OP_SETUPVAR. This is for speed tuning.
Make topenv() inlined.
Clean some redundant code up.
Lazy variable declarations. They are for speed tune.
Simplify conditional expressions. This is for speed tuning.
Merge pull request #1030 from monaka/pr-some-optimizes-to-vm.c-20130318

Some optimizes to vm.c