[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 #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
+}