fix(format): keep a macro-named label line whole

A label whose name is a macro expands into whatever the body is, so the
line's statement structure exists only after expansion; splitting the
label off changed clean input into a different statement sequence.  The
formatter now records the names #define'd above each line and renders
such a label line unsplit, and the crashing input joins the corpus.

Assisted-by: GLM 5.3

format/expansion_test.go

@@ -95,6 +95,26 @@ func TestFormatPreservesExpansionSemantics(t *testing.T) {
 				"\tM(1, R0)\n" +
 				"\tRET\n",
 		},
+		{
+			// A label whose name is a macro: the expansion decides what the
+			// line becomes (here an instruction with several operands), so
+			// the canonical form keeps the line whole instead of splitting
+			// the label off, which would change the statement structure.
+			name: "label named as a macro",
+			src: "#define M B0, C0\n" +
+				"TEXT ·f(SB), $0\n" +
+				"\tM: A00\n" +
+				"\tRET\n",
+		},
+		{
+			// A use before the define: the name is not a macro yet at that
+			// point of the file, so the label splits as any other does.
+			name: "label before the define of its name",
+			src: "TEXT ·f(SB), $0\n" +
+				"\tM: A00\n" +
+				"\tRET\n" +
+				"#define M B0, C0\n",
+		},
 	}
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {