DAGCombiner: don't optimise non-existant litpool load

This particular DAG combine is designed to kick in when both ConstantFPs will
end up being loaded via a litpool, however those nodes have a semi-legal
status, dictated by isFPImmLegal so in some cases there wouldn't have been a
litpool in the first place. Don't try to be clever in those circumstances.

Picked up while merging some AArch64 tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206365 91177308-0d34-0410-b5e6-96231b3b80d8
ARM64: mark x7 as used when an i128 gets shunted onto the stack.

The second half of a split i128 was ending up in x7, which is not a good thing.

This is another part of PR19432.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206366 91177308-0d34-0410-b5e6-96231b3b80d8
ARM64: explicitly ask for Apple NEON syntax so test passes on Linux

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206368 91177308-0d34-0410-b5e6-96231b3b80d8
[Allocator] Nuke to useless functions. The implicit ones are sufficient
here (obviously).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206369 91177308-0d34-0410-b5e6-96231b3b80d8