Implement codegen support for labels and gotos. We now compile:

void test1() {
  foo:
    goto foo;
    goto foo;
    goto foo;
}

void test() {
  goto l;

l:
  ;
}
into:

define void @test1() {
entry:
        br label %foo

foo:            ; preds = %0, %foo, %entry
        br label %foo
                ; No predecessors!
        br label %foo
                ; No predecessors!
}

define void @test() {
entry:
        br label %l

l:              ; preds = %entry
}

llvm-svn: 39524