[AMDGPU] Removed dead code from R600ISelLowering.cpp

[rampitec]

Summary

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.

[RKSimon]

@rampitec This buildbot at least can still reproduce it: http://lab.llvm.org:8011/builders/ppc64le-lld-multistage-test/builds/7216/steps/build-stage1-unified-tree/logs/warnings%20%282%29

[rampitec]

In D69863#1740021, @RKSimon wrote:

@rampitec This buildbot at least can still reproduce it: http://lab.llvm.org:8011/builders/ppc64le-lld-multistage-test/builds/7216/steps/build-stage1-unified-tree/logs/warnings%20%282%29

What version of gcc does it use? It is a clear misreport from gcc, and I don't see the warning from gcc 7.4.0, where the other analyzer correctly detects removed statement as a dead code. I.e. looks like that gcc version is buggy but eventually will be replaced by a more fresh version.

llvm/lib/Target/AMDGPU/R600ISelLowering.cpp

if (ConstantFPSDNode *C = dyn_cast<ConstantFPSDNode>(NewBldVec[i])) {
       } else if (C->isExactlyValue(1.0)) {
         RemapSwizzle[i] = 5; // SEL_1
         NewBldVec[i] = DAG.getUNDEF(MVT::f32);
       }
     }
 
     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());
         RemapSwizzle[i] = j;
         break;
       }
     }
   }