Use dex cache from compilation unit in HInstructionBuilder.

Avoid calling costly ClassLinker::FindDexCache() from
HInstructionBuilder, the dex cache is already available in
the compilation unit. Compiling Nexus 5 boot image on host
under perf(1) shows that the time spent in FindDexCache()
is reduced from about 2% to well under 0.2%, 90% of the
remaining hits coming from ReferenceTypePropagation which
doesn't have access to the compilation unit.

Change-Id: Id9df61c79db9a67e3f12c857f32a4ca80e5420c3