[CodeGen] Fix incorrect uses of getVectorNumElements()

I have fixed up some places in SelectionDAG::getNode() where we
used to assert that the number of vector elements for two types
are the same. I have changed such cases to assert that the
element counts are the same instead. I've added new tests that
exercise the code paths for all the truncations. All the extend
operations are covered by this existing test:

  CodeGen/AArch64/sve-sext-zext.ll

For the ISD::SETCC case I fixed this code path is exercised by
these existing tests:

  CodeGen/AArch64/sve-fcmp.ll
  CodeGen/AArch64/sve-intrinsics-int-compares-with-imm.ll

Differential Revision: https://reviews.llvm.org/D79399