Resubmit "[X86] Adding new LLVM TableGen backend that generates the X86 backend memor...

Resubmit "[X86] Adding new LLVM TableGen backend that generates the X86 backend memory folding tables."

This was reverted due to buildbot breakages and I was not familiar
with this code to investigate it.  But while trying to get a
useful backtrace for the author, it turns out the fix was very
obvious.  Resubmitting this patch as is, and will submit the
fix in a followup so that the fix is not hidden in the larger
CL.

llvm-svn: 304122