[AArch64] `LowerBUILD_VECTOR()`: `NormalizeBuildVector()` might return non-BUILD_VECTOR

As apparent in the newly-added test, provided in:
https://github.com/llvm/llvm-project/commit/cf624b23bc5d5a6161706d1663def49380ff816a#commitcomment-90836329,
we should be more careful with handling wider vectors,
or we will assert later on.