[X86] Move all atom CPUs to the same section of the switch and use...

[X86] Move all atom CPUs to the same section of the switch and use fallthroughs like we do for other CPU generations. NFC

This is prep work to add MOVBE to all Atom CPUs. This instruction didn't come in to the Nehalem/Westmere/SandyBridge/etc. line until later so there's no natural place to overlap the Atom CPUs into that part of the switch.

llvm-svn: 306849