RISC-V: Sanitise NEED_EQ_NE_P case with `riscv_emit_int_compare'

For the NEED_EQ_NE_P `riscv_emit_int_compare' is documented to only emit
EQ or NE comparisons against zero, however it does not catch incorrect
use where a non-equality comparison has been requested and falls through
to the general case then.  Add a safety guard to catch such a case then.

Arguably the NEED_EQ_NE_P case would best be moved into a function of
its own, but let's leave it for a separate cleanup.

	gcc/
	* config/riscv/riscv.cc (riscv_emit_int_compare): Bail out if
	NEED_EQ_NE_P but the comparison is neither EQ nor NE.

gcc/config/riscv/riscv.cc

@@ -3808,6 +3808,7 @@ riscv_emit_int_compare (enum rtx_code *code, rtx *op0, rtx *op1,
 	  *op1 = const0_rtx;
 	  return;
 	}
+      gcc_unreachable ();
     }
 
   if (splittable_const_int_operand (*op1, VOIDmode))