Reapply [GlobalOpt] Remove unreachable blocks before optimizing a function.

This commit reapplies r307215 now that we found out and fixed
the cause of the cfi test failure (in r307871).

llvm-svn: 307920
diff --git a/llvm/test/Transforms/GlobalOpt/pr33686.ll b/llvm/test/Transforms/GlobalOpt/pr33686.ll
new file mode 100644
index 0000000..153a85f
--- /dev/null
+++ b/llvm/test/Transforms/GlobalOpt/pr33686.ll
@@ -0,0 +1,17 @@
+; RUN: opt -S -globalopt %s | FileCheck %s
+
+; CHECK-LABEL: define void @beth
+; CHECK-NEXT:   entry:
+; CHECK-NEXT:   ret void
+; CHEC-NEXT:  }
+
+@glob = external global i16, align 1
+
+define void @beth() {
+entry:
+  ret void
+
+notreachable:
+  %patatino = select i1 undef, i16* @glob, i16* %patatino
+  br label %notreachable
+}