fix(asm): reject the operand-starved riscv64 spellings instead of panicking

Assisted-by: GLM 5.3

asm/assembler_fuzz_test.go

@@ -232,6 +232,11 @@ func FuzzAssembleRISCV64(f *testing.F) {
 	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tCSRRW $0x1000, X5, X6\n\tRET\n")
 	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tSLLI $64, X5, X6\n\tRET\n")
 	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tVLE8V (X10), V32\n\tRET\n")
+	// Operand-starved spellings that used to panic the layout and encode
+	// passes; each must come back as a diagnostic.
+	f.Add("TEXT ·f(SB), $0\n\tJALR\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tROR $3\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tVLE8V (X10)\n\tRET\n")
 	f.Add("#define A A\nA\n")
 
 	f.Fuzz(func(t *testing.T, src string) {