Avoid many "variable 'Sub' set but not used [-Wunused-but-set-variable]" warnings

with gcc-4.6.  The warning is wrong, since Sub *is* used (perhaps gcc is confused
because the use of Sub is constant folded away?), but since it is trivial to avoid,
and massively reduces the amount of warning spew, just workaround the wrong warning.

llvm-svn: 105788