[RegisterBankInfo] Move to statically allocated RegisterBank.

This commit is basically the first step toward what will
RegisterBankInfo look when it gets TableGen'ed.

It introduces a XXXGenRegisterBankInfo.def file that is what TableGen
will issue at some point. Moreover, the RegBanks field in
RegisterBankInfo changed to reflect the static (compile time) aspect of
the information.

llvm-svn: 282131