AMDGPU: Reduce 64-bit lshr by constant to 32-bit

64-bit shifts are very slow on some subtargets.

llvm-svn: 258090
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.h b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.h
index 3792541..920df8f 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.h
+++ b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.h
@@ -69,6 +69,7 @@
 
   SDValue performStoreCombine(SDNode *N, DAGCombinerInfo &DCI) const;
   SDValue performShlCombine(SDNode *N, DAGCombinerInfo &DCI) const;
+  SDValue performSrlCombine(SDNode *N, DAGCombinerInfo &DCI) const;
   SDValue performMulCombine(SDNode *N, DAGCombinerInfo &DCI) const;
   SDValue performCtlzCombine(SDLoc SL, SDValue Cond, SDValue LHS, SDValue RHS,
                              DAGCombinerInfo &DCI) const;