Initial Thumb2 support. Majority of the work is done by David Goodwin. There are

also some contribution from Jim Grosbach, Bob Wilson, and Evan Cheng.

I've done my best to consolidate the patches with those that were done by
Viktor Kutuzov and Anton Korzh from Access Softek, Inc. Let me know if missed
anything. I've completely reorganized the thumb2 td file, made more extensive
uses of multiclass, etc.

Test cases will be contributed later after I re-organize what's in svn first.

llvm-svn: 73965