Merging r353308 and r353383:

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r353308 | tnorthover | 2019-02-06 16:26:35 +0100 (Wed, 06 Feb 2019) | 5 lines

AArch64: enforce even/odd register pairs for CASP instructions.

ARMv8.1a CASP instructions need the first of the pair to be an even register
(otherwise the encoding is unallocated). We enforced this during assembly, but
not CodeGen before.
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r353383 | tnorthover | 2019-02-07 11:35:34 +0100 (Thu, 07 Feb 2019) | 4 lines

AArch64: implement copy for paired GPR registers.

When doing 128-bit atomics using CASP we might need to copy a GPRPair to a
different register, but that was unimplemented up to now.
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llvm-svn: 353822