[dataflow] Don't rescan everything after CSE

[Paolo Bonzini]

This saves ~1% of compilation time.

More ideas that I have, but did not implement are:

1) delay computation of LR/UR until where we did it in the trunk.  fwprop and GCSE do not need it, for example.

2) don't update REG_DEAD/REG_UNUSED notes for patches that don't need it.  Updating them could/should be a changeable flag.

Bootstrapped/regtested i686-pc-linux-gnu.

Paolo
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