[AArch64] Add bitcasts for lowering saturating add/sub and shift intrinsics.

[Lukacma]

This is followup patch to #157680 . In this patch, we are adding explicit bitcasts to floating-point type when lowering saturating add/sub and shift NEON scalar intrinsics using SelectionDAG, so they can be picked up by patterns added in first part of this series. To do that, we have to create new nodes for these intrinsics, which operate on floating-point types and wrapp them in bitcast nodes.

[Lukacma]

This is my first attempt at implementing this part and I would appreciate your feedback on this approach as there are couple of things I am uncertain about here.

Firstly, GlobalISel works fine with original intrinsics and doesn't require custom lowering of them, I decided to leave it be and instead duplicate patterns as you can see. (one which match new node for SDag and the other variant which matches original intrinsic). I am not sure if this is the correct approach or if I should lower to the new node also in GlobalISel.

Secondly, I am not sure if creating new nodes is necessary here as well. I was thinking about abusing intrinsics a bit and adding floating point variants of them. For example sqadd intrinsic is defined like this:

DefaultAttrsIntrinsic<[llvm_anyint_ty], [LLVMMatchType<0>, LLVMMatchType<0>],
                [IntrNoMem]>;

I was thinking about generalizing it to allow it to be used with floating-point types, in which case I could lower into the floating-point variant and wouldn't need to create new ISD nodes. This should also resolve the issue with pattern duplication due to different lowerings, if I am not missing smth. I am not sure though whether such "abuse" is fine (I don't think it is, but it would make my life simpler).