Remove the late DCE in RegisterCoalescer.
Remove the late DCE in RegisterCoalescer.

Dead code and joined copies are now eliminated on the fly, and there is
no need for a post pass.

This makes the coalescer work like other modern register allocator
passes: Code is changed on the fly, there is no pending list of changes
to be committed.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@157132 91177308-0d34-0410-b5e6-96231b3b80d8

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