atomics A.1b: real RMW emission (atomicrmw) + unit test (green)

emitAtomicRmw: LLVMBuildAtomicRMW (binop from RmwKind; signed Min/Max vs
unsigned UMin/UMax from val_ty; singleThread=0; LLVM supplies ABI alignment).
examples/1701 green (add/sub/and/or/xor/min/max return old values, results
verified). Unit test 'emit: atomic rmw (add + signed/unsigned min)' locks
'atomicrmw add' + signed 'min' vs unsigned 'umin'. Suite green (716/0).

src/ir/emit_llvm.test.zig

@@ -248,6 +248,38 @@ test "emit: atomic load/store (seq_cst, aligned)" {
     try std.testing.expect(std.mem.indexOf(u8, ir_str, "align 8") != null);
 }
 
+test "emit: atomic rmw (add + signed/unsigned min)" {
+    const alloc = std.testing.allocator;
+    var module = Module.init(alloc);
+    defer module.deinit();
+
+    var b = Builder.init(&module);
+
+    _ = b.beginFunction(str(&module, "f"), &.{}, .i64);
+    const entry = b.appendBlock(str(&module, "entry"), &.{});
+    b.switchToBlock(entry);
+
+    const si = b.alloca(.i64);
+    const ui = b.alloca(.u64);
+    const five = b.constInt(5, .i64);
+    _ = b.emit(.{ .atomic_rmw = .{ .ptr = si, .operand = five, .val_ty = .i64, .ordering = .seq_cst, .kind = .add } }, .i64);
+    _ = b.emit(.{ .atomic_rmw = .{ .ptr = si, .operand = five, .val_ty = .i64, .ordering = .seq_cst, .kind = .min } }, .i64);
+    _ = b.emit(.{ .atomic_rmw = .{ .ptr = ui, .operand = five, .val_ty = .u64, .ordering = .seq_cst, .kind = .min } }, .u64);
+    b.ret(five, .i64);
+    b.finalize();
+
+    var emitter = LLVMEmitter.init(alloc, &module, "test_rmw", .{});
+    defer emitter.deinit();
+    emitter.emit();
+
+    try std.testing.expect(emitter.verify());
+
+    const ir_str = emitter.dumpToString();
+    try std.testing.expect(std.mem.indexOf(u8, ir_str, "atomicrmw add") != null);
+    try std.testing.expect(std.mem.indexOf(u8, ir_str, "atomicrmw min") != null); // signed i64
+    try std.testing.expect(std.mem.indexOf(u8, ir_str, "atomicrmw umin") != null); // unsigned u64
+}
+
 test "emit: comparison and branch" {
     const alloc = std.testing.allocator;
     var module = Module.init(alloc);