properly detect ssa form

Previously, we would skip ssa construction when
a temporary has a single definition.  This is
only part of the ssa invariant: we must also
check that all uses are dominated by the single
definition.  The new code does this.

In fact, qbe does not store all the dominators
for a block, so instead of walking the idom
linked list we use a rough heuristic and declare
conservatively that B0 dominates B1 when one of
the two conditions is true:

  a. B0 is the start block
  b. B0 is B1

Some measurements on a big file from Michael
Forney show that the code is still as fast as
before this patch.

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