[IR] Bail out for scalable vectors in ShuffleVectorInst::isConcat

Shuffle mask for concat can't be expressed for scalable vectors, so we
should bail out. A test has been added that previously crashed, also
tested isIdentityWithPadding and isIdentityWithExtract where we already
bail out.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D92475