compiler: fix non-int integer constants

Before this change, the compiler could panic with the following message:

    panic: 20 not an Int

That of course doesn't make much sense. But it apparently is expected
behavior, see https://github.com/golang/go/issues/43165 for details.

This commit fixes this issue by converting the constant to an integer if
needed.

testdata/binop.go

@@ -70,6 +70,11 @@ func main() {
 	println(ashrNeg == -1)
 	println(ashrOverflow == 0)
 	println(ashrNegOverflow == -1)
+
+	// fix for a bug in constant numbers
+	println("constant number")
+	x := uint32(5)
+	println(uint32(x) / (20e0 / 1))
 }
 
 var x = true