With fixupCommoning, any node commoned across the split point is stored
into a temp, and occurrences after the split are replaced with a load.

If this happened with a PassThrough node, we would generate BadILOp
because PassThrough claims to have type NoType. In particular, the store
would be BadILOp, and it would appear that a conversion is needed at the
load, and the conversion would also be BadILOp.

While determining which nodes are commoned across the split point, any
PassThrough node encountered with refcount greater than 1 will now be
replaced with a fresh PassThrough node with refcount 1. This way, no
PassThrough node will be found to commoned across the split point.

(cherry picked from commit 7879fa4)