Modify DXILPrepare to emit no-op bitcasts

In supporting opaque pointers we need to re-materialize typed pointers
in bitcode emission. Because of how the value-enumerator pre-allocates
types and instructions we need to insert some no-op bitcasts in the
places that we'll need bitcasts for the pointer types.

Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D122269