[CodeLayout] Fix X1_Y_X2 and Y_X2_X1 testing for jumps from Y (llvm#66592)

The CHECK2 test in code_placement_ext_tsp_large.ll now has the same
result as
the CHECK test: when chain(0,2,3,4,1) is merged with chain(8), the
result is now
chain(0,2,3,4,8,1).

Ideally we should have test coverage for
-ext-tsp-chain-split-threshold=1, but
it seems challenging to craft one. Perhaps the default value of
-ext-tsp-chain-split-threshold can be decreased as the
-ext-tsp-enable-chain-split-along-jumps heuristic is now more powerful.

Author: MaskRay

llvm/lib/Transforms/Utils/CodeLayout.cpp

@@ -850,7 +850,7 @@ class ExtTSPImpl {
 
       // Attach (a part of) ChainPred after the last node of ChainSucc.
       for (JumpT *Jump : ChainSucc->Nodes.back()->OutJumps) {
-        const NodeT *DstBlock = Jump->Source;
+        const NodeT *DstBlock = Jump->Target;
         if (DstBlock->CurChain != ChainPred)
           continue;
         size_t Offset = DstBlock->CurIndex;

llvm/test/CodeGen/X86/code_placement_ext_tsp_large.ll

@@ -81,19 +81,18 @@ define void @func_large() !prof !0 {
 ; CHECK: b7
 ; CHECK: b9
 ;
-; An expected output with chain-split-threshold=1 (disabling splitting) -- the
-; increase of the layout score is smaller, ~7%:
+; An expected output with chain-split-threshold=1 (disabling split point enumeration)
 ;
 ; CHECK2-LABEL: Applying ext-tsp layout
 ; CHECK2:   original  layout score: 9171074274.27
-; CHECK2:   optimized layout score: 9810644873.57
+; CHECK2:   optimized layout score: 10844307310.87
 ; CHECK2: b0
 ; CHECK2: b2
 ; CHECK2: b3
 ; CHECK2: b4
 ; CHECK2: b5
-; CHECK2: b1
 ; CHECK2: b8
+; CHECK2: b1
 ; CHECK2: b6
 ; CHECK2: b7
 ; CHECK2: b9