[InstCombine] Limit a vector demanded elts rule which was producing invalid IR.

The demanded elts rules introduced for GEPs in https://reviews.llvm.org/rL356293 replaced vector constants with undefs (by design).  It turns out that the LangRef disallows such cases when indexing structs.  The right fix is probably to relax the langref requirement, and update other passes to expect the result, but for the moment, limit the transform to avoid compiler crashes.

This should fix https://bugs.llvm.org/show_bug.cgi?id=41624.

llvm-svn: 359633