[InstSimplify] Clarify use of FixedVectorType in SimplifySelectInst
Folding a select of vector constants that include undef elements only
applies to fixed vectors, but there's no earlier check the type is not
scalable so it crashes for scalable vectors. This adds a check so this
optimization is only attempted for fixed vectors.
Reviewed By: sdesmalen
Differential Revision: https://reviews.llvm.org/D92046