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

[Kostya Serebryany]

  >> Do we know the upper limit for size?
  We can set one. E.g. 64G from a single size class.
  >> We could divide both operands by a constant using shifts.
  Exactly!
  I've added a test for this: DISABLED_SizeClassAllocator64GetBlockBegin

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