fix(loong64): FP immediates through R30 and unsigned branch forms

Assisted-by: GLM 5.3

testdata/verify/branchu_loong64.s

@@ -0,0 +1,18 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// branchu exercises the unsigned 16-bit branches in their two- and
+// three-operand spellings: with a single register the toolchain keeps the
+// register-register form against R0 (never taken) rather than a zero-form
+// pseudo.  Byte-parity-checked against go tool asm.
+
+#include "textflag.h"
+
+TEXT ·unsigned(SB), NOSPLIT, $0
+	BLTU R4, done
+	BGEU R5, done
+	BLTU R6, R7, done
+	BGEU R8, R9, done
+
+done:
+	RET