Remove redundant copy in to r126 from stackcall functions.

Change-Id: I9047b8518fe2e3efcb227b0cb44dce7a3b51d8dc

Author: pratikashar

visa/GraphColor.cpp

@@ -6843,7 +6843,12 @@ void GraphColor::stackCallProlog()
 {
     // mov (8) r126.0<1>:ud    r0.0<8;8,1>:ud
     // This sets up the header for oword block r/w used for caller/callee-save
-    // ToDo: check if this move is actually necessary
+
+    // Kernel should've already setup r0 in r126.
+    // Useful data in r126 is expected to be preserved by all functions.
+    if (kernel.fg.getIsStackCallFunc())
+        return;
+
     auto dstRgn = builder.Create_Dst_Opnd_From_Dcl(builder.kernel.fg.scratchRegDcl, 1);
     auto srcRgn = builder.Create_Src_Opnd_From_Dcl(builder.getBuiltinR0(), builder.getRegionStride1());
 
@@ -7803,7 +7808,6 @@ void GlobalRA::addCalleeSavePseudoCode()
     builder.instList.clear();
 }
 
-
 //
 // Insert pseudo operation to store fp at entry and restore before return.
 // Dst of store will be a temp that will run through RA and get an allocation.