Remove readnone from invariant.group.barrier

Remove readnone from invariant.group.barrier

Summary:
Readnone attribute would cause CSE of two barriers with
the same argument, which is invalid by example:

    struct Base {
          virtual int foo() { return 42; }
    };

    struct Derived1 : Base {
          int foo() override { return 50; }
    };

    struct Derived2 : Base {
          int foo() override { return 100; }
    };

    void foo() {
        Base *x = new Base{};
        new (x) Derived1{};
        int a = std::launder(x)->foo();
        new (x) Derived2{};
        int b = std::launder(x)->foo();
    }

Here 2 calls of std::launder will produce @llvm.invariant.group.barrier,
which would be merged into one call, causing devirtualization
to devirtualize second call into Derived1::foo() instead of
Derived2::foo()

Reviewers: chandlerc, dberlin, hfinkel

Subscribers: llvm-commits, rsmith, amharc

Differential Revision: https://reviews.llvm.org/D31531

llvm-svn: 300101