Update the EarlyCSE tests to use opaque pointers. Mainly putting this up for review to get confirmation that it's fine to start migrating tests.
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 these 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.