Add option to enable/disable ibl optimization code in x86_to_x64 mode

_From [piyuske...@gmail.com](https://code.google.com/u/117873943802993320844/) on August 15, 2014 08:50:48_

By default ibl code uses extra available registers when running x86 code on x64 architecture. We need to add an option to enable/disable this optimization.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1514_

Start by locating the x86_to_x64 mode implementation and the IBL optimization code, then inspect how configuration options are defined and consumed. The work is done when an option clearly enables or disables that optimization and its behavior is covered by the relevant existing checks or tests.

Written by the indexing model from the issue text.