[VPlan] Use vector.body as header name in VPlan native path.

This brings the VPlan block naming in line with the naming of the
generated basic blocks.

llvm/lib/Transforms/Vectorize/VPlanHCFGBuilder.cpp

@@ -264,6 +264,7 @@ VPRegionBlock *PlainCFGBuilder::buildPlainCFG() {
   }
   // Create empty VPBB for Loop H so that we can link PH->H.
   VPBlockBase *HeaderVPBB = getOrCreateVPBB(TheLoop->getHeader());
+  HeaderVPBB->setName("vector.body");
   // Preheader's predecessors will be set during the loop RPO traversal below.
   PreheaderVPBB->setOneSuccessor(HeaderVPBB);
 

llvm/unittests/Transforms/Vectorize/VPlanHCFGTest.cpp

@@ -106,11 +106,11 @@ compound=true
     label="\<x1\> TopRegion"
     N1 [label =
       "entry:\l" +
-      "Successor(s): for.body\l"
+      "Successor(s): vector.body\l"
     ]
     N1 -> N2 [ label=""]
     N2 [label =
-      "for.body:\l" +
+      "vector.body:\l" +
       "  WIDEN-PHI ir\<%indvars.iv\> = phi ir\<0\>, ir\<%indvars.iv.next\>\l" +
       "  EMIT ir\<%arr.idx\> = getelementptr ir\<%A\> ir\<%indvars.iv\>\l" +
       "  EMIT ir\<%l1\> = load ir\<%arr.idx\>\l" +
@@ -118,8 +118,8 @@ compound=true
       "  EMIT store ir\<%res\> ir\<%arr.idx\>\l" +
       "  EMIT ir\<%indvars.iv.next\> = add ir\<%indvars.iv\> ir\<1\>\l" +
       "  EMIT ir\<%exitcond\> = icmp ir\<%indvars.iv.next\> ir\<%N\>\l" +
-      "Successor(s): for.body, for.end\l" +
-      "CondBit: ir\<%exitcond\> (for.body)\l"
+      "Successor(s): vector.body, for.end\l" +
+      "CondBit: ir\<%exitcond\> (vector.body)\l"
     ]
     N2 -> N2 [ label="T"]
     N2 -> N3 [ label="F"]