[FuncSpec] Prevent assertion failure when no store value is found

[jpenix-quic]

Summary

If the only user of the Alloca argument provided to getPromotableAlloca()
is the same as the Call argument, StoreValue is never set and results
in an assertion failure that isa<> was used on a nullptr when passed into
getCandidateConstant().

This was originally seen when trying to build SPEC 2006 416.gamess using
flang with lto enabled.

llvm/lib/Transforms/IPO/FunctionSpecialization.cpp

if (auto *Store = dyn_cast<StoreInst>(User)) {
       if (StoreValue || Store->isVolatile())
         return nullptr;
       StoreValue = Store->getValueOperand();
       continue;
     }
     // Bail if there is any other unknown usage.
     return nullptr;
   }
+
+  if (!StoreValue)
+    return nullptr;
+
   return getCandidateConstant(StoreValue);
 }
 
 // A constant stack value is an AllocaInst that has a single constant
 // value stored to it. Return this constant if such an alloca stack value
 // is a function argument.
 Constant *FunctionSpecializer::getConstantStackValue(CallInst *Call,
                                                      Value *Val) {

llvm/test/Transforms/FunctionSpecialization/compiler-crash-promote-alloca.ll

+; RUN: opt -S --passes="ipsccp<func-spec>" -force-function-specialization < %s | FileCheck %s
+
+; Tests that `bar` has been specialized and that the compiler did not crash
+; while attempting to promote the alloca in `entry`.
+; CHECK: bar.1
+
+@block = internal global [8 x i8] zeroinitializer, align 1
+
+define dso_local void @entry() {
+  %1 = alloca i32, align 4
+  call void @foo(ptr nonnull %1)
+  ret void
+}
+
+define internal void @foo(ptr nocapture readnone %0) {
+  %2 = alloca i32, align 4
+  call void @bar(ptr nonnull %2, ptr nonnull @block)
+  call void @bar(ptr nonnull %2, ptr nonnull getelementptr inbounds ([8 x i8], ptr @block, i64 0, i64 4))
+  ret void
+}
+
+define internal void @bar(ptr nocapture readonly %0, ptr nocapture readonly %1) {
+  %3 = load i32, ptr %0, align 4
+  %4 = load i32, ptr %1, align 4
+  ret void
+}
+