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

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.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D92607