[FastISel] Some long overdue spring cleaning of FastISel.

Things got a little bit messy over the years and it is time for a little bit
spring cleaning.

This first commit is focused on the FastISel base class itself. It doxyfies all
comments, C++11fies the code where it makes sense, renames internal methods to
adhere to the coding standard, and clang-formats the files.

Reviewed by Eric

llvm-svn: 217060