Track register ages more accurately.

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144601 91177308-0d34-0410-b5e6-96231b3b80d8

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