Change FindEarliestCallSeqEnd (used by libcall insertion) to use a set to

avoid revisiting nodes more than once.  This eliminates a source of
potentially exponential behavior.  For a small function in 191.fma3d
(hexah_stress_divergence_), this speeds up isel from taking > 20mins to
taking 0.07s.

llvm-svn: 22680