RISC-V: Fallback for instructions longer than 64b

We don't support instructions longer than 64-bits yet.  Still, we can
modify validate_riscv_insn function to prevent unexpected behavior by
limiting the "length" of an instruction to 64-bit (or less).

gas/ChangeLog:

	* config/tc-riscv.c (validate_riscv_insn): Fix function
	description comment based on current spec.  Limit instruction
	length up to 64-bit for now.  Make sure that required_bits does
	not corrupt even if unsigned long long is longer than 64-bit.

gas/config/tc-riscv.c

@@ -1109,7 +1109,8 @@ arg_lookup (char **s, const char *const *array, size_t size, unsigned *regnop)
 
 /* For consistency checking, verify that all bits are specified either
    by the match/mask part of the instruction definition, or by the
-   operand list. The `length` could be 0, 4 or 8, 0 for auto detection.  */
+   operand list. The `length` could be the actual instruction length or
+   0 for auto-detection.  */
 
 static bool
 validate_riscv_insn (const struct riscv_opcode *opc, int length)
@@ -1120,11 +1121,13 @@ validate_riscv_insn (const struct riscv_opcode *opc, int length)
   insn_t required_bits;
 
   if (length == 0)
-    insn_width = 8 * riscv_insn_length (opc->match);
-  else
-    insn_width = 8 * length;
+    length = riscv_insn_length (opc->match);
+  /* We don't support instructions longer than 64-bits yet.  */
+  if (length > 8)
+    length = 8;
+  insn_width = 8 * length;
 
-  required_bits = ~0ULL >> (64 - insn_width);
+  required_bits = ((insn_t)~0ULL) >> (64 - insn_width);
 
   if ((used_bits & opc->match) != (opc->match & required_bits))
     {