[MachineOutliner][AArch64] Improve checks for stack instructions

If we know that we'll definitely save LR to a register, there's no reason to
pre-check whether or not a stack instruction is unsafe to fix up.

This makes it so that we check for that condition before mapping instructions.

This allows us to outline more, since we don't pessimise as many instructions.

Also update some tests, since we outline more.

llvm-svn: 348081