[c++11] Use overdrive instead of virtual.

Also removes a few methods that were never called.
Re-add support for -mc-relax-all to vex_llc.

Was removed to get the code compiling.
Promote zero-extended loads of i1.
Add getSelectionDAGInfo implementation to rvexSubtarget.
Increase rvex DFA array size.

Fixes a problem with data overwrites.