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Draft: Aarch64 popcount

Alex Mason requested to merge Axman6/ghc:aarch64-popcount into master

Some initial, possibly controversial, work to add support for popcount using the built in cnt instruction. This instruction requires access to vector registers, so this PR adds some initial support for them. AArch64's vector registers are a little strange, they are both integer and float registers, but are really just the float registers renamed.

The pop count is very simple, the source is moved into the low bytes of a vector reg, cnt (which counts the set bits in each byte and returns the result in each byte) modifies that register, the bytes are summed, and the result is moved into the dest register (which is what clang produces for __builtin_popcountll etc.).

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