[GlobalOpt] Fix unreachable ifunc globalopt crash (#157332) (#157593)

Also fixes (#131488)

Unreachable case is triggering `Callees.empty()` assert. Since this was
[originally
](02bd5a7#diff-06aba0dac2a263dc14297a15655291d5506b760f54a736385bcf3208f83df843R2524)
a `continue` anyway, have applied that as a fix and added a test case.
Please let me know if there's a better way.

Not sure who/how to get folks to review, tagging a few people (apologies
if you're not the right person/this is the wrong way to do it, please
let me know what to do in future if so)

@labrinea @dtcxzyw @nikic @fhahn

Author: Arghnews

llvm/lib/Transforms/IPO/GlobalOpt.cpp

@@ -2551,7 +2551,8 @@ static bool OptimizeNonTrivialIFuncs(
         }))
       continue;
 
-    assert(!Callees.empty() && "Expecting successful collection of versions");
+    if (Callees.empty())
+      continue;
 
     LLVM_DEBUG(dbgs() << "Statically resolving calls to function "
                       << Resolver->getName() << "\n");

llvm/test/Transforms/GlobalOpt/resolve-indirect-ifunc.ll

@@ -0,0 +1,11 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt --passes=globalopt -o - -S < %s | FileCheck %s
+
+define ptr @f1() {
+; CHECK-LABEL: define ptr @f1() {
+; CHECK-NEXT:    unreachable
+;
+  unreachable
+}
+
+@i1 = ifunc void(), ptr @f1