feat(asm): take the loong64 register-pair spellings the toolchain parses

Assisted-by: GLM 5.3 Flash

testdata/verify/pairs_loong64.s

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+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Differential kernel for the loong64 register-pair spellings: a top-level
+// colon between two registers splits the operand in two with the halves
+// swapped, so R4:R5 encodes exactly as R5, R4.  The integer multiply and
+// divide families, the FP pair and the vector pair, each the way
+// GOARCH=loong64 go tool asm takes them.
+
+#include "textflag.h"
+
+TEXT ·pairs(SB), NOSPLIT, $0-0
+	MULV   R4 : R5, R6
+	MULV   R4 : R5
+	MULV   R6, R4 : R5
+	DIVV   R4 : R5, R6
+	DIVVU  R4 : R5, R6
+	MULHV  R4 : R5, R6
+	MULHVU R4 : R5, R6
+	ADDV   R4 : R5, R6
+	ADDW   R4 : R5, R6
+	AND    R4 : R5, R6
+	OR     R4 : R5, R6
+	XOR    R4 : R5, R6
+	MOVV   R4 : R5
+	MULD   F4 : F5, F6
+	VADDW  V1 : V2, V3
+	RET