ReleaseNotes: matching wide stores (r362472)

  `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 pattern match wide scalar values stored by a succession of

  narrow stores. For example, Clang will compile the following function that

  writes a 32-bit value in big-endian order in a portable manner:

  .. code-block:: c

      void write32be(unsigned char *dst, uint32_t x) {

        dst[0] = x >> 24;

        dst[1] = x >> 16;

        dst[2] = x >> 8;

        dst[3] = x >> 0;

      }

  into the x86_64 code below:

  .. code-block:: asm

   write32be:

           bswap   esi

           mov     dword ptr [rdi], esi

           ret

  (The corresponding read patterns have been matched since LLVM 5.)

* 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