[Local] Update the comment for removeUnreachableBlocks.

It referenced a wrong function name, and didn't mention what the
second argument did. This should be slightly more accurate now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307425 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed