[CodeGen] In narrowExtractedVectorLoad bail out for scalable vectors

In narrowExtractedVectorLoad there is an optimisation that tries to
combine extract_subvector with a narrowing vector load. At the moment
this produces warnings due to the incorrect calls to
getVectorNumElements() for scalable vector types. I've got this
working for scalable vectors too when the extract subvector index
is a multiple of the minimum number of elements. I have added a
new variant of the function:

  MachineFunction::getMachineMemOperand

that copies an existing MachineMemOperand, but replaces the pointer
info with a null version since we cannot currently represent scaled
offsets.

I've added a new test for this particular case in:

  CodeGen/AArch64/sve-extract-subvector.ll

Differential Revision: https://reviews.llvm.org/D83950