Trace back through sibling copies to hoist spills and find rematerializable defs.

After live range splitting, an original value may be available in multiple
registers. Tracing back through the registers containing the same value, find
the best place to insert a spill, determine if the value has already been
spilled, or discover a reaching def that may be rematerialized.

This is only the analysis part. The information is not used for anything yet.

llvm-svn: 127698