This commit fixes a really fun bug with inst combine where you have a
vector-of-pointers and only one element of this vector is used.
InstCombine correctly notes that an element is not used, and then will
go back through a gep into that vector-of-pointers and set all vector
elements to undef. This is fine in all cases except that the langref
(and a ton of places in the optimizer) requires that indexing into a
struct has the same index for each element of the vector-of-pointers.
To fix the bug I've just made the gep/undef propagation check that the
current type being indexed into is not a struct when doing the undef