AstGen: fix @breakpoint ZIR

Previously it relied on the breakpoint ZIR instruction being void, but
that's no longer how things work.

src/AstGen.zig

@@ -3894,11 +3894,11 @@ fn builtinCall(
             return rvalue(gz, scope, rl, result, node);
         },
         .breakpoint => {
-            const result = try gz.add(.{
+            _ = try gz.add(.{
                 .tag = .breakpoint,
                 .data = .{ .node = gz.astgen.decl.nodeIndexToRelative(node) },
             });
-            return rvalue(gz, scope, rl, result, node);
+            return rvalue(gz, scope, rl, .void_value, node);
         },
         .import => {
             const target = try expr(gz, scope, .none, params[0]);

test/stage2/cbe.zig

@@ -280,6 +280,15 @@ pub fn addCases(ctx: *TestContext) !void {
             \\}
         , "");
 
+        // If expression with breakpoint that does not get hit
+        case.addCompareOutput(
+            \\export fn main() c_int {
+            \\    var x: i32 = 1;
+            \\    if (x != 1) @breakpoint();
+            \\    return 0;
+            \\}
+        , "");
+
         // Switch expression
         case.addCompareOutput(
             \\export fn main() c_int {