[ValueTypes] Assert if changeVectorElementType is called on a simple type with...

[ValueTypes] Assert if changeVectorElementType is called on a simple type with an extended element type.

Previously we would use the extended implementation, but
the extended implementation requires the vector type to be extended
so that we can access the LLVMContext. In theory we could
detect this case and use the context from the element type instead,
but since I know of no cases hitting this in practice today
I've done the simplest thing.

Also add asserts to several extended EVT functions that assume
LLVMTy is non-null.

Follow from discussion in D97036

Reviewed By: pengfei

Differential Revision: https://reviews.llvm.org/D97070