[SVE][CodeGen] Fix DAGCombiner::ForwardStoreValueToDirectLoad for scalable vectors

In DAGCombiner::ForwardStoreValueToDirectLoad I have fixed up some
implicit casts from TypeSize -> uint64_t and replaced calls to
getVectorNumElements() with getVectorElementCount(). There are some
simple cases of forwarding that we can definitely support for
scalable vectors, i.e. when the store and load are both scalable
vectors and have the same size. I have added tests for the new
code paths here:

  CodeGen/AArch64/sve-forward-st-to-ld.ll

Differential Revision: https://reviews.llvm.org/D87098