gas: arm -mwarn-syms duplicates

arm gas is only supposed to warn once per symbol for -mwarn-syms, but
doesn't because the str_hash_find added with commit 629310abec88
always returns NULL.  That's so because the str_hash_insert inserts a
NULL value for the key,value pair.  Let str_hash_insert do the job
instead.

	* config/tc-arm.c (arm_tc_equal_in_insn): Correct already_warned
	logic.
	* testsuite/gas/arm/pr18347.s: Modify to generate duplicate
	warning without this patch.

gas/config/tc-arm.c

diff --git a/gas/config/tc-arm.c b/gas/config/tc-arm.c
index 2e6d175..68fd689 100644
--- a/gas/config/tc-arm.c
+++ b/gas/config/tc-arm.c
@@ -28120,11 +28120,8 @@
 	    already_warned = str_htab_create ();
 	  /* Only warn about the symbol once.  To keep the code
 	     simple we let str_hash_insert do the lookup for us.  */
-	  if (str_hash_find (already_warned, nbuf) == NULL)
-	    {
-	      as_warn (_("[-mwarn-syms]: Assignment makes a symbol match an ARM instruction: %s"), name);
-	      str_hash_insert (already_warned, nbuf, NULL, 0);
-	    }
+	  if (str_hash_insert (already_warned, nbuf, NULL, 0) == NULL)
+	    as_warn (_("[-mwarn-syms]: Assignment makes a symbol match an ARM instruction: %s"), name);
 	}
       else
 	free (nbuf);

gas/testsuite/gas/arm/pr18347.s

diff --git a/gas/testsuite/gas/arm/pr18347.s b/gas/testsuite/gas/arm/pr18347.s
index 591d69b..64a16cb 100644
--- a/gas/testsuite/gas/arm/pr18347.s
+++ b/gas/testsuite/gas/arm/pr18347.s
@@ -1,5 +1,6 @@
     MOV r1, r0
     LDR =garbage // no destination register
+    LDR =garbage // and it should only warn once
     MOV r2, r3
 
 	// The warning should only be triggered by a "foo = bar"