Fixed some error handling for invalid if statements. Started if codegen.

test/parse/else_if_after_else.golden

@@ -0,0 +1,4 @@
+test/else_if_after_else.ink:6,6: error: 'else if' without 'if'
+        } else if 0 > 200 {
+          ^^^^
+

test/parse/float_if_cond.golden

@@ -0,0 +1,6 @@
+(program	
+ (fun vertex vs_main -> float4 (@position)
+  [(:= pos float3 (@position))] 
+  (if 1
+      (return (float4 pos 1))) 
+  (return (float4 0))))

test/parse/if_cond_assign.golden

@@ -0,0 +1,6 @@
+(program	
+ (fun vertex vs_main -> float4 (@position)
+  [(:= pos float3 (@position))] 
+  (if (= 0 100)
+      (return (float4 pos 1))) 
+  (return (float4 0))))

test/parse/if_if_if.golden

@@ -0,0 +1,4 @@
+test/if_if_if.ink:2,3: error: Expected expression after 'if'.
+    if if if 0 > 100 {
+       ^^
+