x64: rename brk to int3, and MIR to interrupt

src/arch/x86_64/CodeGen.zig

@@ -3755,8 +3755,10 @@ fn airArg(self: *Self, inst: Air.Inst.Index) !void {
 
 fn airBreakpoint(self: *Self) !void {
     _ = try self.addInst(.{
-        .tag = .brk,
-        .ops = undefined,
+        .tag = .interrupt,
+        .ops = (Mir.Ops{
+            .flags = 0b00,
+        }).encode(),
         .data = undefined,
     });
     return self.finishAirBookkeeping();

src/arch/x86_64/Emit.zig

@@ -178,7 +178,7 @@ pub fn lowerMir(emit: *Emit) InnerError!void {
 
             .@"test" => try emit.mirTest(inst),
 
-            .brk => try emit.mirBrk(),
+            .interrupt => try emit.mirInterrupt(inst),
             .nop => try emit.mirNop(),
 
             .call_extern => try emit.mirCallExtern(inst),
@@ -225,8 +225,14 @@ fn fixupRelocs(emit: *Emit) InnerError!void {
     }
 }
 
-fn mirBrk(emit: *Emit) InnerError!void {
-    return lowerToZoEnc(.brk, emit.code);
+fn mirInterrupt(emit: *Emit, inst: Mir.Inst.Index) InnerError!void {
+    const tag = emit.mir.instructions.items(.tag)[inst];
+    assert(tag == .interrupt);
+    const ops = Mir.Ops.decode(emit.mir.instructions.items(.ops)[inst]);
+    switch (ops.flags) {
+        0b00 => return lowerToZoEnc(.int3, emit.code),
+        else => return emit.fail("TODO handle variant 0b{b} of interrupt instruction", .{ops.flags}),
+    }
 }
 
 fn mirNop(emit: *Emit) InnerError!void {
@@ -1074,7 +1080,7 @@ const Tag = enum {
     push,
     pop,
     @"test",
-    brk,
+    int3,
     nop,
     imul,
     mul,
@@ -1278,7 +1284,7 @@ inline fn getOpCode(tag: Tag, enc: Encoding, is_one_byte: bool) ?OpCode {
         .zo => return switch (tag) {
             .ret_near => OpCode.oneByte(0xc3),
             .ret_far => OpCode.oneByte(0xcb),
-            .brk => OpCode.oneByte(0xcc),
+            .int3 => OpCode.oneByte(0xcc),
             .nop => OpCode.oneByte(0x90),
             .syscall => OpCode.twoByte(0x0f, 0x05),
             .cbw => OpCode.oneByte(0x98),

src/arch/x86_64/Mir.zig

@@ -342,8 +342,9 @@ pub const Inst = struct {
         /// TODO handle more cases
         @"test",
 
-        /// Breakpoint
-        brk,
+        /// Breakpoint  form:
+        ///       0b00  int3
+        interrupt,
 
         /// Nop
         nop,