[EarlyCSE] Update tests to use opaque pointers (NFC)

Update the EarlyCSE tests to use opaque pointers.

Worth noting that this leaves some bitcast ptr to ptr instructions
in the input IR behind which are no longer necessary. This is
because these use numbered instructions, so it's hard to drop them
in an automated fashion (as it would require renumbering all other
instructions as well). I'm leaving that as a problem for another day.

The test updates have been performed using
https://gist.github.com/nikic/98357b71fd67756b0f064c9517b62a34.

Differential Revision: https://reviews.llvm.org/D127278