[SVE] Ignore scalable vectors in InterleavedLoadCombinePass

I have changed the pass so that we ignore shuffle vectors with
scalable vector types, and replaced VectorType with FixedVectorType
in the rest of the pass. I couldn't think of an easy way to test
this change, since for scalable vectors we shouldn't be using
shufflevectors for interleaving. This change fixes up some
type size assert warnings I found in the following test:

  CodeGen/AArch64/sve-intrinsics-int-arith-imm.ll

Differential Revision: https://reviews.llvm.org/D79700