[RISC-V][PR target/123283] Wrap naked REG operands with a USE.

I was in the process of testing this patch when Andreas filed PR123283.

What's going on is we have patterns in sync.md which have naked operands:

(define_insn "subword_atomic_fetch_strong_<atomic_optab>"
  [(set (match_operand:SI 0 "register_operand" "=&r")   ;; old value at mem
    (match_operand:SI 1 "memory_operand" "+A"))        ;; mem location
   (set (match_dup 1)
    (unspec_volatile:SI
      [(any_atomic:SI (match_dup 1)
             (match_operand:SI 2 "arith_operand" "rI")) ;; value for op
       (match_operand:SI 3 "const_int_operand")]           ;; model
     UNSPEC_SYNC_OLD_OP_SUBWORD))
    (match_operand:SI 4 "arith_operand" "rI")          ;; mask
    (match_operand:SI 5 "arith_operand" "rI")          ;; not_mask
    (clobber (match_scratch:SI 6 "=&r"))               ;; tmp_1
    (clobber (match_scratch:SI 7 "=&r"))]              ;; tmp_2

Note carefully operands #4 and #5 and the fact they are a toplevel construct as
opposed to being an operand of another RTX.  That's a no-no.  They need to be
wrapped with a USE.

I spot-checked sync.md and found a few more instances.   Fixing the set I found
fixed the testsuite regressions I was seeing and also fixes the mis-compilation
of libgo.  Bootstrapped and regression tested on my BPI and Pioneer.  It's also
clean on the riscv64-elf and riscv32-elf targets in my tester.

	PR target/123283
gcc/
	* config/riscv/sync.md (subword_atomic_fetch_strong_nand): Add
	USEs for naked operands that might be pseudos.
	(subword_atomic_fetch_strong_<atomic_optab>): Likewise.
	(subword_atomic_exchange_strong): Likewise.
	(subword_atomic_cas_strong): Likewise.

Author: Jeff Law

gcc/config/riscv/sync.md

@@ -210,8 +210,8 @@
 		     (match_operand:SI 2 "arith_operand" "rI")) ;; value for op
 	   (match_operand:SI 3 "const_int_operand")]		   ;; model
 	 UNSPEC_SYNC_OLD_OP_SUBWORD))
-    (match_operand:SI 4 "arith_operand" "rI")		   ;; mask
-    (match_operand:SI 5 "arith_operand" "rI")		   ;; not_mask
+    (use (match_operand:SI 4 "arith_operand" "rI"))	   ;; mask
+    (use (match_operand:SI 5 "arith_operand" "rI"))	   ;; not_mask
     (clobber (match_scratch:SI 6 "=&r"))			   ;; tmp_1
     (clobber (match_scratch:SI 7 "=&r"))]			   ;; tmp_2
   "TARGET_ZALRSC && TARGET_INLINE_SUBWORD_ATOMIC"
@@ -277,8 +277,8 @@
 			   (match_operand:SI 2 "arith_operand" "rI")))    ;; value for op
 	   (match_operand:SI 3 "const_int_operand")]			  ;; mask
 	 UNSPEC_SYNC_OLD_OP_SUBWORD))
-    (match_operand:SI 4 "arith_operand" "rI")				  ;; mask
-    (match_operand:SI 5 "arith_operand" "rI")				  ;; not_mask
+    (use (match_operand:SI 4 "arith_operand" "rI"))		  ;; mask
+    (use (match_operand:SI 5 "arith_operand" "rI"))		  ;; not_mask
     (clobber (match_scratch:SI 6 "=&r"))				  ;; tmp_1
     (clobber (match_scratch:SI 7 "=&r"))]				  ;; tmp_2
   "TARGET_ZALRSC && TARGET_INLINE_SUBWORD_ATOMIC"
@@ -512,7 +512,7 @@
 	  [(match_operand:SI 2 "arith_operand" "rI")	 ;; value
 	   (match_operand:SI 3 "const_int_operand")]	 ;; model
       UNSPEC_SYNC_EXCHANGE_SUBWORD))
-    (match_operand:SI 4 "arith_operand" "rI")	 ;; not_mask
+    (use (match_operand:SI 4 "arith_operand" "rI"))	 ;; not_mask
     (clobber (match_scratch:SI 5 "=&r"))]		 ;; tmp_1
   "TARGET_ZALRSC && TARGET_INLINE_SUBWORD_ATOMIC"
   {
@@ -785,10 +785,10 @@
 	(unspec_volatile:SI [(match_operand:SI 2 "reg_or_0_operand" "rJ")  ;; expected value
 			     (match_operand:SI 3 "arith_operand" "rI")] ;; desired value
 	 UNSPEC_COMPARE_AND_SWAP_SUBWORD))
-	(match_operand:SI 4 "const_int_operand")			   ;; model
-	(match_operand:SI 5 "arith_operand" "rI")			   ;; mask
-	(match_operand:SI 6 "arith_operand" "rI")			   ;; not_mask
-	(clobber (match_scratch:SI 7 "=&r"))]				   ;; tmp_1
+	(match_operand:SI 4 "const_int_operand")		;; model
+	(use (match_operand:SI 5 "arith_operand" "rI"))		;; mask
+	(use (match_operand:SI 6 "arith_operand" "rI"))		;; not_mask
+	(clobber (match_scratch:SI 7 "=&r"))]			;; tmp_1
   "TARGET_ZALRSC && TARGET_INLINE_SUBWORD_ATOMIC"
   {
     return "1:\;"