Fix comparison of mixed signedness

177774 broke the lld-x86_64-darwin11 builder; error:
error: comparison of integers of different signs: 'int' and 'size_type' (aka 'unsigned long')
  for (SI = 0; SI < Scavenged.size(); ++SI)
               ~~ ^ ~~~~~~~~~~~~~~~~

Fix this by making SI also unsigned.

llvm-svn: 177780