Add a "seen blocks" cache to LVI to avoid a linear scan over the whole cache...

Add a "seen blocks" cache to LVI to avoid a linear scan over the whole cache just to remove no blocks from the maps.

-15% on ARMDisassembler.cpp (Release build).  It's not that great to add another
layer of caching to the caching-heavy LVI but I don't see a better way.

llvm-svn: 145770