[IndVarSimplify] Bail out if not a branch in maybeFloatingPointRecurrence

clang-riscv-gauntlet buildbot was previously failing. Conservatively
bail out if not handling a branch when looking for floating-point
recurrences, we may wish to support select and branch in the future.

Fixes: https://lab.llvm.org/buildbot/#/builders/210/builds/5908.

Author: antoniofrighetto

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

@@ -306,6 +306,9 @@ maybeFloatingPointRecurrence(Loop *L, PHINode *PN) {
   // The branch block must be in the loop and one of the successors must be out
   // of the loop.
   auto *BI = dyn_cast<BranchInst>(Compare->user_back());
+  if (!BI)
+    return std::nullopt;
+
   assert(BI->isConditional() && "Can't use fcmp if not conditional");
   if (!L->contains(BI->getParent()) ||
       (L->contains(BI->getSuccessor(0)) && L->contains(BI->getSuccessor(1))))