[FuncSpec] Avoid crashing when SwitchInst doesn't see ConstantInt

D150464 updated the cost model for function specialization. Unfortunately, this
also crashes when trying to build stage2 LLD with thinLTO and assertions. It looks
like the issue is caused by a mishandling of the Constant in a SwitchInst since the
Constant cannot always be assumed to safely casted to a ConstantInt. In the case
of the crash, Constant was a ConstantExpr which triggered the assertion.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D154159

llvm/lib/Transforms/IPO/FunctionSpecialization.cpp

@@ -190,7 +190,10 @@ Cost InstCostVisitor::estimateSwitchInst(SwitchInst &I) {
   if (I.getCondition() != LastVisited->first)
     return 0;
 
-  auto *C = cast<ConstantInt>(LastVisited->second);
+  auto *C = dyn_cast<ConstantInt>(LastVisited->second);
+  if (!C)
+    return 0;
+
   BasicBlock *Succ = I.findCaseValue(C)->getCaseSuccessor();
   // Initialize the worklist with the dead basic blocks. These are the
   // destination labels which are different from the one corresponding

llvm/test/Transforms/FunctionSpecialization/function-specialization-constant-expression6.ll

@@ -0,0 +1,22 @@
+; RUN: opt -passes="ipsccp<func-spec>" -force-specialization -S < %s
+; Check that we don't crash when SwitchInst Constant is not ConstantInt.
+
+@S = external constant [1 x i8]
+
+define i1 @foo() {
+entry:
+  %tmp = call i32 @bar(ptr @S)
+  ret i1 0
+}
+
+define i32 @bar(ptr %arg) {
+entry:
+  %magicptr = ptrtoint ptr %arg to i64
+  switch i64 %magicptr, label %bb2 [
+    i64 0, label %bb1
+  ]
+bb1:
+  ret i32 0
+bb2:
+  ret i32 1
+}