cgen: simplify for in range

vlib/v/gen/cgen.v

@@ -715,15 +715,12 @@ fn (mut g Gen) write_defer_stmts() {
 fn (mut g Gen) for_in(it ast.ForInStmt) {
 	if it.is_range {
 		// `for x in 1..10 {`
-		i := g.new_tmp_var()
+		i := if it.val_var == '_' { g.new_tmp_var() } else { c_name(it.val_var) }
 		g.write('for (int $i = ')
 		g.expr(it.cond)
 		g.write('; $i < ')
 		g.expr(it.high)
 		g.writeln('; $i++) {')
-		if it.val_var != '_' {
-			g.writeln('\tint ${c_name(it.val_var)} = $i;')
-		}
 		g.stmts(it.stmts)
 		g.writeln('}')
 	} else if it.kind == .array {

vlib/v/tests/for_loops_test.v

@@ -18,6 +18,14 @@ fn test_for_char_in_range() {
 	assert sum == 6
 }
 
+fn test_for_blank_in_range() {
+	mut sum := 0
+	for _ in 1 .. 3 {
+		sum++
+	}
+	assert sum == 2
+}
+
 fn test_for_char_complex() {
 	mut sum := 0
 	for char := 0; char < nums.len; char++ {