Fix 1 + distribution having reverse ordered points

Fixes #595

packages/squiggle-lang/__tests__/Distributions/AlgebraicShapeCombination_test.res

@@ -14,4 +14,16 @@ describe("Combining Continuous and Discrete Distributions", () => {
     ), // Multiply distribution by -1
     true,
   )
+  makeTest(
+    "keep order of xs when first number is discrete and adding",
+    AlgebraicShapeCombination.isOrdered(
+      AlgebraicShapeCombination.combineShapesContinuousDiscrete(
+        #Add,
+        {xs: [0., 1.], ys: [1., 1.]},
+        {xs: [1.], ys: [1.]},
+        ~discretePosition=First,
+      ),
+    ), // 1 + distribution
+    true,
+  )
 })

packages/squiggle-lang/src/rescript/Distributions/PointSetDist/AlgebraicShapeCombination.res

@@ -214,7 +214,7 @@ let combineShapesContinuousDiscrete = (
         // When this operation is flipped (like 1 - normal(5, 2)) then the
         // x axis coordinates would all come out the wrong order. So we need
         // to fill them out in the opposite direction
-        let index = discretePosition == First ? t1n - 1 - i : i
+        let index = discretePosition == First && op == #Subtract ? t1n - 1 - i : i
         Belt.Array.set(
           dxyShape,
           index,