[AArch64] Fix memmodel index in aarch64_store_exclusive_pair

Found via an rtx checking failure.

2019-09-23  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/atomics.md (aarch64_store_exclusive_pair): Fix
	memmodel index.

From-SVN: r276052

gcc/ChangeLog

+2019-09-23  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* config/aarch64/atomics.md (aarch64_store_exclusive_pair): Fix
+	memmodel index.
+
 2019-09-23  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>
 
 	PR ipa/91835

gcc/config/aarch64/atomics.md

@@ -752,7 +752,7 @@
 	  UNSPECV_SX))]
   ""
   {
-    enum memmodel model = memmodel_from_int (INTVAL (operands[3]));
+    enum memmodel model = memmodel_from_int (INTVAL (operands[4]));
     if (is_mm_relaxed (model) || is_mm_consume (model) || is_mm_acquire (model))
       return "stxp\t%w0, %x2, %x3, %1";
     else