ReleaseNotes: omitting range checks for switches with unreachable defaults

  `bug 42763 <https://bugs.llvm.org/show_bug.cgi?id=42763>_` and

  `post commit discussion <http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20190422/646945.html>_`.  

* LLVM will now omit range checks for jump tables when lowering switches with

  unreachable default destination. For example, the switch dispatch in the C++

  code below

  .. code-block:: c

     int g(int);

     enum e { A, B, C, D, E };

     int f(e x, int y, int z) {

         switch(x) {

             case A: return g(y);

             case B: return g(z);

             case C: return g(y+z);

             case D: return g(x-z);

             case E: return g(x+z);

         }

     }

  will result in the following x86_64 machine code when compiled with Clang.

  This is because falling off the end of a non-void function is undefined

  behaviour in C++, and the end of the function therefore being treated as

  unreachable:

  .. code-block:: asm

   _Z1f1eii:

           mov     eax, edi

           jmp     qword ptr [8*rax + .LJTI0_0]

Changes to the LLVM IR

----------------------