Fix #15305 (#15311)

* Fix #15305

* Fix test

compiler/parser.nim

@@ -2242,8 +2242,7 @@ proc parseStmt(p: var Parser): PNode =
           break
         p.hasProgress = false
         if p.tok.tokType in {tkElse, tkElif}:
-          parMessage(p, errInvalidIndentation)
-          getTok(p)
+          break # Allow this too, see tests/parser/tifexprs
 
         result.add complexOrSimpleStmt(p)
         if not p.hasProgress and p.tok.tokType == tkEof: break

tests/parser/tifexprs.nim

@@ -0,0 +1,12 @@
+discard """
+  output: '''
+1
+'''
+"""
+
+var a, b: int
+let x = if a > b:
+    0
+    else: 1
+
+echo x