[LazyValueInfo] Stop inserting overdefined values into ValueCache to

reduce memory usage.

Previously, LazyValueInfoCache inserted overdefined lattice values into
both ValueCache and OverDefinedCache. This wasn't necessary and was
causing LazyValueInfo to use an excessive amount of memory in some cases.

This patch changes LazyValueInfoCache to insert overdefined values only
into OverDefinedCache. The memory usage decreases by 70 to 75% when one
of the files in llvm is compiled.

rdar://problem/11388615

Differential revision: http://reviews.llvm.org/D15391

llvm-svn: 255320