[DSE,MemorySSA] Increase walker limit a bit.

This slightly bumps the walker limit so that it covers more cases while
not increasing compile-time too much:
http://llvm-compile-time-tracker.com/compare.php?from=0fc1c2b51ba0cfb9145139af35be638333865251&to=91144a50ea4fa82c0c877e77784f60371640b263&stat=instructions