RegisterCoalescer: Turn some impossible conditions into asserts

This is a fixed version of reverted r225500. It fixes the too early
if() continue; of the last patch and adds a comment to the unorthodox
loop.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225652 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed