[AMDGPU] Removed dead code from R600ISelLowering.cpp

This was added to inhibit a warning from gcc 7.3 according to
the comment. However, it triggers warning from PVS. In addition
I cannot reproduce it with gcc 7.4 and I also cannot reproduce
it with gcc 7.3 using compiler explorer.

Differential Revision: https://reviews.llvm.org/D69863

Author: rampitec

llvm/lib/Target/AMDGPU/R600ISelLowering.cpp

@@ -1715,12 +1715,7 @@ static SDValue CompactSwizzlableVector(
 
     if (NewBldVec[i].isUndef())
       continue;
-    // Fix spurious warning with gcc 7.3 -O3
-    //    warning: array subscript is above array bounds [-Warray-bounds]
-    //    if (NewBldVec[i] == NewBldVec[j]) {
-    //        ~~~~~~~~~~~^
-    if (i >= 4)
-      continue;
+
     for (unsigned j = 0; j < i; j++) {
       if (NewBldVec[i] == NewBldVec[j]) {
         NewBldVec[i] = DAG.getUNDEF(NewBldVec[i].getValueType());