This pr updates `DXILPrepare` and `DXILTranslateMetadata` by moving all
the removal of metadata from `DXILPrepare` to `DXILTranslateMetadata` to
have a more consistent definition of what each pass is doing.
It restricts the `DXILPrepare` to only update function attributes and
insert bitcasts, and moves the removal of metadata to
`DXILTranslateMetadata` so that all manipulation of metadata is done in
a single pass.
0fd330dfe3