Calling life_analysis from other optimizers (i.e. VRP)

[John Wehle]

It would be nice if VRP could use global_live_at_start to determine
if it should bother propagating value range information for a given
register.  This would (among other things) cut down on the amount
of memory used by VRP.  Any problem with VRP calling:

  life_analysis (get_insns (), NULL, PROP_KILL_DEAD_CODE | PROP_SCAN_DEAD_CODE);

before starting its main loop?  Any other thoughts / suggestions?

-- John
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