Emit correct code when making a ConstantPool entry for a vector

Emit correct code when making a ConstantPool entry for a vector
constant whose component type is not a legal type for the target.
(If the target ConstantPool cannot handle this type either, it has
an opportunity to merge elements.  In practice any target with
8-bit bytes must support i8 *as data*).  7320806 (partial).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@86751 91177308-0d34-0410-b5e6-96231b3b80d8