[SystemZ] Split up PerformDAGCombine. [NFC]

This function is already a bit too long, and I'm about to make it worse.

llvm-svn: 274191
diff --git a/llvm/lib/Target/SystemZ/SystemZISelLowering.h b/llvm/lib/Target/SystemZ/SystemZISelLowering.h
index bb3b718..9a8adee 100644
--- a/llvm/lib/Target/SystemZ/SystemZISelLowering.h
+++ b/llvm/lib/Target/SystemZ/SystemZISelLowering.h
@@ -545,6 +545,13 @@
                          bool Force) const;
   SDValue combineTruncateExtract(const SDLoc &DL, EVT TruncVT, SDValue Op,
                                  DAGCombinerInfo &DCI) const;
+  SDValue combineSIGN_EXTEND(SDNode *N, DAGCombinerInfo &DCI) const;
+  SDValue combineMERGE(SDNode *N, DAGCombinerInfo &DCI) const;
+  SDValue combineSTORE(SDNode *N, DAGCombinerInfo &DCI) const;
+  SDValue combineEXTRACT_VECTOR_ELT(SDNode *N, DAGCombinerInfo &DCI) const;
+  SDValue combineJOIN_DWORDS(SDNode *N, DAGCombinerInfo &DCI) const;
+  SDValue combineFP_ROUND(SDNode *N, DAGCombinerInfo &DCI) const;
+  SDValue combineBSWAP(SDNode *N, DAGCombinerInfo &DCI) const;
 
   // If the last instruction before MBBI in MBB was some form of COMPARE,
   // try to replace it with a COMPARE AND BRANCH just before MBBI.