[AMDGPU] Add a test case for #98063

Add a test case to demonstrate broken IR caused by #98063 "[AMDGPU] Do
not use original PHIs in coercion chains" before it was reverted.

Author: jayfoad

llvm/test/CodeGen/AMDGPU/amdgpu-late-codegenprepare.ll

@@ -93,3 +93,58 @@ define amdgpu_kernel void @constant_from_inttoptr() {
   store i8 %load, ptr addrspace(1) undef
   ret void
 }
+
+define void @broken_phi() {
+; GFX9-LABEL: @broken_phi(
+; GFX9-NEXT:  bb:
+; GFX9-NEXT:    br label [[BB1:%.*]]
+; GFX9:       bb1:
+; GFX9-NEXT:    [[I:%.*]] = phi <4 x i8> [ <i8 1, i8 1, i8 1, i8 1>, [[BB:%.*]] ], [ [[I8:%.*]], [[BB7:%.*]] ]
+; GFX9-NEXT:    br i1 false, label [[BB3:%.*]], label [[BB2:%.*]]
+; GFX9:       bb2:
+; GFX9-NEXT:    br label [[BB3]]
+; GFX9:       bb3:
+; GFX9-NEXT:    [[I4:%.*]] = phi <4 x i8> [ zeroinitializer, [[BB2]] ], [ [[I]], [[BB1]] ]
+; GFX9-NEXT:    br i1 false, label [[BB7]], label [[BB5:%.*]]
+; GFX9:       bb5:
+; GFX9-NEXT:    [[I6:%.*]] = call <4 x i8> @llvm.smax.v4i8(<4 x i8> [[I4]], <4 x i8> zeroinitializer)
+; GFX9-NEXT:    br label [[BB7]]
+; GFX9:       bb7:
+; GFX9-NEXT:    [[I8]] = phi <4 x i8> [ zeroinitializer, [[BB5]] ], [ zeroinitializer, [[BB3]] ]
+; GFX9-NEXT:    br label [[BB1]]
+;
+; GFX12-LABEL: @broken_phi(
+; GFX12-NEXT:  bb:
+; GFX12-NEXT:    br label [[BB1:%.*]]
+; GFX12:       bb1:
+; GFX12-NEXT:    [[I:%.*]] = phi <4 x i8> [ <i8 1, i8 1, i8 1, i8 1>, [[BB:%.*]] ], [ [[I8:%.*]], [[BB7:%.*]] ]
+; GFX12-NEXT:    br i1 false, label [[BB3:%.*]], label [[BB2:%.*]]
+; GFX12:       bb2:
+; GFX12-NEXT:    br label [[BB3]]
+; GFX12:       bb3:
+; GFX12-NEXT:    [[I4:%.*]] = phi <4 x i8> [ zeroinitializer, [[BB2]] ], [ [[I]], [[BB1]] ]
+; GFX12-NEXT:    br i1 false, label [[BB7]], label [[BB5:%.*]]
+; GFX12:       bb5:
+; GFX12-NEXT:    [[I6:%.*]] = call <4 x i8> @llvm.smax.v4i8(<4 x i8> [[I4]], <4 x i8> zeroinitializer)
+; GFX12-NEXT:    br label [[BB7]]
+; GFX12:       bb7:
+; GFX12-NEXT:    [[I8]] = phi <4 x i8> [ zeroinitializer, [[BB5]] ], [ zeroinitializer, [[BB3]] ]
+; GFX12-NEXT:    br label [[BB1]]
+;
+bb:
+  br label %bb1
+bb1:
+  %i = phi <4 x i8> [ <i8 1, i8 1, i8 1, i8 1>, %bb ], [ %i8, %bb7 ]
+  br i1 false, label %bb3, label %bb2
+bb2:
+  br label %bb3
+bb3:
+  %i4 = phi <4 x i8> [ zeroinitializer, %bb2 ], [ %i, %bb1 ]
+  br i1 false, label %bb7, label %bb5
+bb5:
+  %i6 = call <4 x i8> @llvm.smax.v4i8(<4 x i8> %i4, <4 x i8> zeroinitializer)
+  br label %bb7
+bb7:
+  %i8 = phi <4 x i8> [ zeroinitializer, %bb5 ], [ zeroinitializer, %bb3 ]
+  br label %bb1
+}