Keep track of the last instruction to define each register individually
instead of per DomainValue.  This lets us track more accurately when a
register was last written.

Also track register ages across basic blocks.  When entering a new
basic block, use the least stale predecessor def as a worst case
estimate for register age.

The register age is used to arbitrate between conflicting domains. The
most recently defined register wins.

llvm-svn: 144601