This giant patch speeds up Type::isSized(). Before, this would have to search

large nested types over and over again to determine if they are sized or not.
Now, isSized() is able to make snap decisions about all concrete types, which
are a common occurance (and includes all primitives).

On 177.mesa, this speeds up DSE from 39.5s -> 21.3s and GCSE from
13.2s -> 11.3s, reducing gccas time from 80s -> 61s (this is a debug build).

DSE and GCSE are still too slow on this testcase, but this is a simple
improvement.

llvm-svn: 19800