[InstCombine] improve debug value names; NFC

The use of 'tmp' can trigger warnings from the update_test_checks.py
script. That's evidence of a flaw in the script's logic, but we
can always do better than naming variables 'tmp' in LLVM too.

The phi test file should be updated with auto-generated regex CHECK
lines, so it isn't affected by cosmetic diffs, but I don't have
time to do that right now.