SelectionDAG switch lowering: use 'unsigned' to count destination popularity

SwitchInst::getNumCases() returns unsinged, so using uint64_t to count cases
seems unnecessary.

Also fix a missing CHECK in the test case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224393 91177308-0d34-0410-b5e6-96231b3b80d8