introduce if-else expression, implement case in propagation when consequence and alternative are equal, adjust tests

zokrates_core_test/tests/tests/panics/conditional_bound_throw.zok

@@ -0,0 +1,10 @@
+def throwing_bound<N>(u32 x) -> u32:
+    assert(x == N)
+    return 1
+
+// this compiles: the conditional, even though it can throw, has a constant compile-time value of `1`
+// However, the assertions are still checked at runtime, which leads to panics without branch isolation.
+def main(u32 x):
+  for u32 i in 0..if x == 0 then throwing_bound::<0>(x) else throwing_bound::<1>(x) fi do
+  endfor
+  return

zokrates_core_test/tests/tests/panics/conditional_bound_throw_no_isolation.json

@@ -0,0 +1,51 @@
+{
+    "entry_point": "./tests/tests/panics/conditional_bound_throw.zok",
+    "curves": ["Bn128"],
+	"tests": [
+        {
+            "input": {
+                "values": [
+                    "0"
+                ]
+            },
+            "output": {
+				"Err": {
+					"UnsatisfiedConstraint": {
+						"left": "0",
+						"right": "1"
+					}
+				}
+            }
+        },
+        {
+            "input": {
+                "values": [
+                    "1"
+                ]
+            },
+            "output": {
+				"Err": {
+					"UnsatisfiedConstraint": {
+						"left": "1",
+						"right": "0"
+					}
+				}
+            }
+        },
+        {
+            "input": {
+                "values": [
+                    "2"
+                ]
+            },
+            "output": {
+				"Err": {
+					"UnsatisfiedConstraint": {
+						"left": "2",
+						"right": "0"
+					}
+				}
+            }
+        }
+    ]
+}