Fix SSA rewrite for nested loops.

From the test case, the slice of the CFG that is interesting for the bug
is

25
|
v
30
|
v
31<-+
|   |
v   |
34--+

1. In block 25, we have a Phi candidate for %f with arguments
   %47 = Phi[%float_0, %0]. This merges %float_0 and a yet unknown
   argument from the external loop backedge.
2. We are now processing block 34:
   i. The load %35 = OpLoad %f triggers a Phi candidate to be placed in
      block 31.
  ii. The Phi candidate %50 = Phi needs two arguments. The one coming
      from block 30 is %47. But the one coming from block 34 (which we
      are now processing and have marked sealed), finds %50 itself as
      the reaching def for %f.
3. This wrongfully marks %50 as a copy-of Phi, which ultimately makes
   both %47 and %50 copy-of Phis that get eliminated.