[GVNSink] Convert tests to opaque pointers (NFC)

Keeping bitcasts in the common_bitcast() test, otherwise sinking
no longer occurs there. GVNSink seems to have a silly cost model
where sinking just the stores is considered non-profitable, but
sinking the stores and bitcasts (which are free...) is profitable.

As this is not a default-enabled pass, and this problem would exist
without opaque pointers as well, I'm not trying to do anything
about this.