patch 9.0.1107: float constant not recognized as float

Problem:    Float constant not recognized as float.
Solution:   Check the vartype instead of comparing with t_float.
            (closes #11754)

src/testdir/test_vim9_expr.vim

@@ -2045,6 +2045,13 @@ def Test_expr9_number()
       assert_equal(6, 0x6)
       assert_equal(15, 0xf)
       assert_equal(255, 0xff)
+
+      const INFTY = 1.0 / 0.0
+      def Test()
+        assert_equal(1, isinf(INFTY))
+        assert_equal(-1, isinf(-INFTY))
+      enddef
+      Test()
   END
   v9.CheckDefAndScriptSuccess(lines)
 enddef