[AArch64][SVE] Add lowering for llvm.maxnum|minnum for scalable type.

[huihuiz]

Summary

LLVM intrinsic llvm.maxnum|minnum is overloaded intrinsic, can be used on any
floating-point or vector of floating-point type.
This patch extends current infrastructure to support scalable vector type.

This patch also fix a warning message of incorrect use of EVT::getVectorNumElements()
for scalable type, when DAGCombiner trying to split scalable vector.

[sdesmalen]

llvm/test/CodeGen/AArch64/sve-fp.ll

597

Can you also add at least one case for each intrinsic (maxnum and minnum) where the vector needs splitting? e.g. using either nxv16f16, nxv8f32 or nxv4f64.

[huihuiz]

Thanks for the feedback!
Test cases for vector splitting added. Also fix the warning message caused by extracting subvector.

[sdesmalen]

Thanks for adding the tests and fixing the warnings, LGTM!