amdgcn: use unsigned extend for lshiftrt

This fixes a wrong-code logic error in a previous patch.
Detected by gcc.c-torture/execute/pr53645-2.c.

	gcc/
	* config/gcn/gcn-valu.md (v<expander><mode>3): Fix unsignedp.

(cherry picked from commit b8db70e1f1cea312888470e1911c7b5e32333c61)

gcc/config/gcn/gcn-valu.md

@@ -2113,7 +2113,7 @@
   ""
   {
     enum {ashift, lshiftrt, ashiftrt};
-    bool unsignedp = (<code> == ashift || <code> == ashiftrt);
+    bool unsignedp = (<code> == lshiftrt);
     rtx insi1 = gen_reg_rtx (<VnSI>mode);
     rtx insi2 = gen_reg_rtx (<VnSI>mode);
     rtx outsi = gen_reg_rtx (<VnSI>mode);