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[AtomicExpandPass]: Add a hook for custom cmpxchg expansion in IR

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[AtomicExpandPass]: Add a hook for custom cmpxchg expansion in IR

This involves changing the shouldExpandAtomicCmpXchgInIR interface, but I have
updated the in-tree backends using this hook (ARM, AArch64, Hexagon) so they
will see no functional change. Previously this hook returned bool, but it now
returns AtomicExpansionKind.

This hook allows targets to select how a given cmpxchg is to be expanded.
D48131 uses this to expand part-word cmpxchg to a target-specific intrinsic.

See my associated RFC for more info on the motivation for this change
http://lists.llvm.org/pipermail/llvm-dev/2018-June/123993.html.

Differential Revision: https://reviews.llvm.org/D48130

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