[PATCH] [sanitizer] Fix overflow in SizeClassAllocator64::GetChunkIdx().

[Kostya Serebryany]

  r181989 has the fix.

  I've remeasured 64-bit div vs 32-bit div on SizeClassAllocator64MetadataStress.
  It is significant: 7.5 seconds vs 4.5 seconds

http://llvm-reviews.chandlerc.com/D797