[test][InstCombine] Update some more test cases to use opaque pointers

These tests cases were converted using the script at
https://gist.github.com/nikic/98357b71fd67756b0f064c9517b62a34,
but there was also a need to re-run update_test_checks.py (impacting
nonnull/dereferencable attributes).

Differential Revision: https://reviews.llvm.org/D135095