[StructurizeCFG] nested-if zerocost hoist bugfix (#155408)

When zero cost instructions are hoisted, the simplifyHoistedPhi function
was setting incoming phi values which were not dominating the use
causing runtime failure. This was set to poison by rebuildSSA function.
This commit fixes the issue.

Author: VigneshwarJ

llvm/lib/Transforms/Scalar/StructurizeCFG.cpp

@@ -997,7 +997,8 @@ void StructurizeCFG::simplifyHoistedPhis() {
         continue;
 
       OtherPhi->setIncomingValue(PoisonValBBIdx, V);
-      Phi->setIncomingValue(i, OtherV);
+      if (DT->dominates(OtherV, Phi))
+        Phi->setIncomingValue(i, OtherV);
     }
   }
 }

llvm/test/CodeGen/AMDGPU/structurize-hoist.ll

@@ -189,12 +189,11 @@ define void @test_nested_if(ptr %ptr, i32 %val, i1 %cond) {
 ; GFX900-NEXT:    s_mov_b64 s[10:11], -1
 ; GFX900-NEXT:    s_xor_b64 s[4:5], s[6:7], -1
 ; GFX900-NEXT:    s_mov_b64 s[12:13], s[6:7]
-; GFX900-NEXT:    ; implicit-def: $vgpr3
+; GFX900-NEXT:    s_waitcnt vmcnt(0) lgkmcnt(0)
+; GFX900-NEXT:    v_mov_b32_e32 v3, v4
 ; GFX900-NEXT:    s_and_saveexec_b64 s[8:9], s[4:5]
 ; GFX900-NEXT:    s_cbranch_execz .LBB3_4
 ; GFX900-NEXT:  ; %bb.1: ; %if
-; GFX900-NEXT:    s_waitcnt vmcnt(0) lgkmcnt(0)
-; GFX900-NEXT:    v_mov_b32_e32 v3, v4
 ; GFX900-NEXT:    s_and_saveexec_b64 s[12:13], s[4:5]
 ; GFX900-NEXT:    s_cbranch_execz .LBB3_3
 ; GFX900-NEXT:  ; %bb.2: ; %if_2
@@ -230,7 +229,6 @@ define void @test_nested_if(ptr %ptr, i32 %val, i1 %cond) {
 ; GFX900-NEXT:    s_or_b64 exec, exec, s[8:9]
 ; GFX900-NEXT:    s_and_saveexec_b64 s[6:7], s[4:5]
 ; GFX900-NEXT:    s_or_b64 exec, exec, s[6:7]
-; GFX900-NEXT:    s_waitcnt vmcnt(0) lgkmcnt(0)
 ; GFX900-NEXT:    flat_store_dword v[0:1], v4
 ; GFX900-NEXT:    s_waitcnt vmcnt(0) lgkmcnt(0)
 ; GFX900-NEXT:    s_setpc_b64 s[30:31]

llvm/test/Transforms/StructurizeCFG/hoist-zerocost.ll

@@ -172,7 +172,7 @@ define void @test_nested_if(ptr %ptr, i32 %val, i1 %cond) {
 ; CHECK-NEXT:    [[A16:%.*]] = icmp slt i32 [[VAL]], 255
 ; CHECK-NEXT:    br i1 [[COND_INV]], label %[[THEN_2:.*]], label %[[FLOW1:.*]]
 ; CHECK:       [[FLOW]]:
-; CHECK-NEXT:    [[TMP0:%.*]] = phi i32 [ [[TMP2:%.*]], %[[FLOW1]] ], [ poison, %[[ENTRY]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = phi i32 [ [[TMP2:%.*]], %[[FLOW1]] ], [ [[A_ELSE]], %[[ENTRY]] ]
 ; CHECK-NEXT:    [[TMP1:%.*]] = phi i1 [ [[TMP3:%.*]], %[[FLOW1]] ], [ [[COND]], %[[ENTRY]] ]
 ; CHECK-NEXT:    br i1 [[TMP1]], label %[[ELSE:.*]], label %[[MERGE:.*]]
 ; CHECK:       [[THEN_2]]: