AArch64/ARM64: copy byval implementation from AArch64.

It's not actually used to handle C or C++ ABI rules on ARM64, but could well be
emitted by other language front-ends, so it's as well to have a sensible
implementation.
AArch64/ARM64: port BSL logic from AArch64 & enable test.

I enhanced it a little in the process. The decision shouldn't really be beased
on whether a BUILD_VECTOR is a splat: any set of constants will do the job
provided they're related in the correct way.

Also, the BUILD_VECTOR could be any operand of the incoming AND nodes, so it's
best to check for all 4 possibilities rather than assuming it'll be the RHS.
AArch64/ARM64: emit all vector FP comparisons as such.

ARM64 was scalarizing some vector comparisons which don't quite map to
AArch64's compare and mask instructions. AArch64's approach of sacrificing a
little efficiency to emulate them with the limited set available was better, so
I ported it across.

More "inspired by" than copy/paste since the backend's internal expectations
were a bit different, but the tests were invaluable.
ARM64: nick some AArch64 patterns for extract/insert -> INS.

Tests will be committed shortly when all optimisations needed to
support AArch64's neon-copy.ll file are supported.
ARM64: spot a vector_shuffle that maps to INS and expand.

Tests will be coming very shortly when all the optimisations needed to
support AArch64's neon-copy.ll file are committed.
ARM64: implement cunning optimisation from AArch64

A vector extract followed by a dup can become a single instruction even if the
types don't match. AArch64 handled this in ISelLowering, but a few reasonably
simple patterns can take care of it in TableGen, so that's where I've put it.
AArch64/ARM64: spot a greater variety of concat_vector operations.

Code mostly copied from AArch64, just tidied up a trifle and plumbed
into the ARM64 way of doing things.

This also enables the AArch64 tests which inspired the previous
untested commits.
AArch64/ARM64: port atomics test to ARM64.

Covers quite a few extra instructions (like any of the max/min ones
which were broken until recently on ARM64).
Revert r206565 (and r206566 which updated tests).

This commit was attributed to a different person from the person who
posted the patch to the list, and the person who posted it the list
claimed when they did that they were not the author, but that the author
was yet a third person. I don't know what is going on here, but
reverting until the attribution is clear and the author has explicitly
contributed the patch.

Also, the review hasn't really involved any of the MC maintainers and
that seems questionable too.