"minimal" SSA form (in other words, it doesn't insert dead PHIs).  This
speeds up the mem2reg pass very significantly because it doesn't have to
do a lot of frivolous work in many common cases.

In the 252.eon function I have been playing with, this doesn't even insert
the 120 PHI nodes that it used to which were trivially dead (in the process
of promoting 356 alloca instructions overall).  This speeds up the mem2reg
pass from 1.2459s to 0.1284s.  More significantly, the DCE pass used to take
2.4138s to remove the 120 dead PHI nodes that mem2reg constructed, now it
takes 0.0134s (which is the time to scan the function and decide that there
is nothing dead).  So overall, on this one function, we speed things up a
total of 3.5179s, which is a 24.8x speedup!  :)

This change is tested by the Mem2Reg/2003-10-05-DeadPHIInsertion.ll test,
which now passes.

llvm-svn: 8884