Rewrite optimization management (see #75)

[doe300]

• Introduces optimization levels which enable a set of optimizations.
• Also allows to enable/disable single optimizations.
• The optimization parameter (e.g. threshold to combine literal loads) can be set via the argument to enable the optimization (see help)
• Updates the help to reflect these changes
• Enables selection of optimizations for TestVC4C (similar to what is in VC4C)

Closes #75

[nomaddo]

It's ok for me to disable or enable optimizations by your commit. But, for setting parameters, it's not convinient.

Now only the threshold of combineLoadLiterals can be adjusted by passing parameter. In this implementation, --fcombine-loads=1000 can set the threshold. It works, but in near future

• Some parameter will be shared by a few optimizations
• Some optimizations will need multiple parameters to adjust

Then, the name of option for that will be strange for users. I think there is no beautiful way to solve the problem. It's better to do like previous way.

[nomaddo]

I like the way how to add parameters adjusted by compilation options except one point I wrote. After solving this, we can merge it.

[nomaddo]

Seems good to me +1