Constant::IsThreadDependent(): Use dyn_cast<Constant> instead of cast

It turns out that the operands of a Constant are not always themselves
Constant. For example, one of the operands of BlockAddress is
BasicBlock, which is not a Constant.

This should fix the dragonegg-x86_64-linux-gcc-4.6-test build which
broke in r168037.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168147 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/GlobalOpt/blockaddress.ll

@@ -0,0 +1,20 @@
+; RUN: opt < %s -globalopt -S | FileCheck %s
+
+@x = internal global i8* zeroinitializer
+
+define void @f() {
+; CHECK: @f
+
+; Check that we don't hit an assert in Constant::IsThreadDependent()
+; when storing this blockaddress into a global.
+
+  store i8* blockaddress(@g, %here), i8** @x, align 8
+  ret void
+}
+
+define void @g() {
+; CHECK: @g
+
+here:
+  ret void
+}