[X86] Remove X86Subtarget::isAtom and replace with tuning flags

[RKSimon]

We still have special case handling for Atom (Bonnell) CPUs, behind a ST->isAtom() check - some of these might be entirely redundant (disabling vector interleaving might be better now with decent cost tables?), while others should be replaced with tuning flags:

• Disable promotion of i16 comparisons
• Use ILP scheduling
• Vector Interleaving

This would rid us of the last remaining CPU specific feature flag (ProcIntelAtom)

[llvmbot]

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