[PlaceSafepoints] Misc. minor cleanups; NFC

These changes are aimed at bringing PlaceSafepoints up to code with the
LLVM coding guidelines:

 - Fix variable naming
 - Use DenseSet instead of std::set
 - Remove dead code
 - Minor local code simplifications

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@259112 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed