AArch64/ARM64: port BSL logic from AArch64 & enable test.

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206569 91177308-0d34-0410-b5e6-96231b3b80d8