Automerge: [SLP]Fix a crash when comparing phis from unreachable blocks

Need to check if the block is reachable before comparing phis from it to
avoid compiler crash when requesting node.

Fixes report in llvm/llvm-project#110529 (comment)

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -5724,6 +5724,10 @@ BoUpSLP::getReorderingData(const TreeEntry &TE, bool TopToBottom) {
 
     auto CompareByBasicBlocks = [&](BasicBlock *BB1, BasicBlock *BB2) {
       assert(BB1 != BB2 && "Expected different basic blocks.");
+      if (!DT->isReachableFromEntry(BB1))
+        return false;
+      if (!DT->isReachableFromEntry(BB2))
+        return true;
       auto *NodeA = DT->getNode(BB1);
       auto *NodeB = DT->getNode(BB2);
       assert(NodeA && "Should only process reachable instructions");

llvm/test/Transforms/SLPVectorizer/AArch64/unreachable-blocks-with-phis.ll

@@ -0,0 +1,43 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S --passes=slp-vectorizer -mtriple=aarch64  < %s | FileCheck %s
+
+define void @test() {
+; CHECK-LABEL: define void @test() {
+; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:    [[TMP0:%.*]] = load <2 x i32>, ptr null, align 1
+; CHECK-NEXT:    br label %[[IF_END:.*]]
+; CHECK:       [[IF_THEN:.*]]:
+; CHECK-NEXT:    br label %[[IF_END]]
+; CHECK:       [[IF_END]]:
+; CHECK-NEXT:    [[TMP1:%.*]] = phi <2 x i32> [ [[TMP0]], %[[ENTRY]] ], [ poison, %[[IF_THEN]] ]
+; CHECK-NEXT:    [[TMP2:%.*]] = extractelement <2 x i32> [[TMP1]], i32 0
+; CHECK-NEXT:    store i32 [[TMP2]], ptr null, align 1
+; CHECK-NEXT:    br label %[[TRAP:.*]]
+; CHECK:       [[BB3:.*:]]
+; CHECK-NEXT:    [[TMP4:%.*]] = extractelement <2 x i32> [[TMP1]], i32 1
+; CHECK-NEXT:    store i32 [[TMP4]], ptr null, align 1
+; CHECK-NEXT:    ret void
+; CHECK:       [[TRAP]]:
+; CHECK-NEXT:    unreachable
+;
+entry:
+  %g_2197.real32.pre = load i32, ptr null, align 1
+  %g_2197.imag33.pre = load i32, ptr getelementptr inbounds nuw ({ i32, i32 }, ptr null, i32 0, i32 1), align 1
+  br label %if.end
+
+if.then:
+  br label %if.end
+
+if.end:
+  %g_2197.imag33 = phi i32 [ %g_2197.imag33.pre, %entry ], [ 0, %if.then ]
+  %g_2197.real32 = phi i32 [ %g_2197.real32.pre, %entry ], [ 0, %if.then ]
+  store i32 %g_2197.real32, ptr  null, align 1
+  br label %trap
+
+0:
+  store i32 %g_2197.imag33, ptr  null, align 1
+  ret void
+
+trap:
+  unreachable
+}