Fix a bug in the SSA builder.

The build would leave behind phis with incompatible input types
(for example float and int). We need another dead phi run after
the type propagation to ensure all such phis are dead.

Change-Id: I6ef1da725c7d4a1ebaf6b52dd7eb0c7bacd261b2
diff --git a/test/459-dead-phi/expected.txt b/test/459-dead-phi/expected.txt
new file mode 100644
index 0000000..ba66466
--- /dev/null
+++ b/test/459-dead-phi/expected.txt
@@ -0,0 +1 @@
+0.0