drnim: phi nodes for 'if' statements (#13990)

drnim/tests/tphi.nim

@@ -0,0 +1,23 @@
+discard """
+  nimout: '''tphi.nim(9, 10) Warning: BEGIN [User]
+tphi.nim(22, 10) Warning: END [User]'''
+  cmd: "drnim $file"
+  action: "compile"
+"""
+import std/logic
+{.push staticBoundChecks: defined(nimDrNim).}
+{.warning: "BEGIN".}
+
+proc testAsgn(y: int) =
+  var a = y
+  if a > 0:
+    if a > 3:
+      a = a + 2
+    else:
+      a = a + 1
+    {.assert: a > 1.}
+
+testAsgn(3)
+
+{.warning: "END".}
+{.pop.}