[CodeGen][AArch64] Bail out in performConcatVectorsCombine for scalable vectors

I tried to exercise the existing combine patterns in performConcatVectorsCombine
for scalable vectors and at the moment it doesn't seem possible. Parts of
the code currently assume we're dealing with fixed-width vectors with calls
to getVectorNumElements(), therefore I've decided to simply bail out early
for scalable vectors.

Added a test here to show that we don't crash when attempting to combine
truncate + concat:

  CodeGen/AArch64/concat_vector-truncate-combine.ll

Differential Revision: https://reviews.llvm.org/D114600