This aims to fix the crash in #168495, my combine rule was
missing a check that the source vector was in fact a vector. This then
caused the legality check to fail in this example as the concat was
trying to concat a non vector.
I have also gated the bitcast of the concat to only work on non-scalable
vectors as the mutation calls `getNumElements` which crashes when called
on a scalable vector.
Fixes#168495
58e6d02aa2