JIT: optimize stelem (ref) like we optimize stelem.ref

[AndyAyersMS]

In particular, avoid the array store check when storing null or values from
the same array.

Addresses first part of #14574.

[AndyAyersMS]

@briansull PTAL
cc @dotnet/jit-contrib

As expected, diffs are in generic code in sorting or swapping methods. jit-diffs shows some size increases; the store element helper does the bounds check and computes the element address while the write barrier just does the store.

Total bytes of diff: 168 (0.00 % of base)
    diff is a regression.
Total byte diff includes 0 bytes from reconciling methods
        Base had    0 unique methods,        0 unique bytes
        Diff had    0 unique methods,        0 unique bytes
Top file regressions by size (bytes):
         122 : System.Private.CoreLib.dasm (0.00 % of base)
          45 : System.Collections.dasm (0.04 % of base)
           1 : System.Collections.Immutable.dasm (0.00 % of base)
3 total files with size differences (0 improved, 3 regressed), 127 unchanged.
Top method regessions by size (bytes):
          45 : System.Collections.dasm - System.Collections.Generic.SortedSet`1[__Canon][System.__Canon]:.ctor(ref,ref):this
          35 : System.Private.CoreLib.dasm - System.Collections.Generic.GenericArraySortHelper`2[__Canon,__Canon][System.__Canon,System.__Canon]:InsertionSort(ref,ref,int,int)
          32 : System.Private.CoreLib.dasm - System.Collections.Generic.ArraySortHelper`2[__Canon,__Canon][System.__Canon,System.__Canon]:InsertionSort(ref,ref,int,int,ref)
          24 : System.Private.CoreLib.dasm - System.Collections.Generic.GenericArraySortHelper`1[__Canon][System.__Canon]:InsertionSort(ref,int,int)
          14 : System.Private.CoreLib.dasm - System.Collections.Generic.GenericArraySortHelper`2[__Canon,__Canon][System.__Canon,System.__Canon]:DownHeap(ref,ref,int,int,int)
Top method improvements by size (bytes):
         -14 : System.Private.CoreLib.dasm - System.Collections.Generic.ArraySortHelper`2[__Canon,__Canon][System.__Canon,System.__Canon]:SwapIfGreaterWithItems(ref,ref,ref,int,int)
          -4 : System.Private.CoreLib.dasm - System.Collections.Generic.ArraySortHelper`1[__Canon][System.__Canon]:SwapIfGreater(ref,ref,int,int)
          -4 : System.Private.CoreLib.dasm - System.Collections.Generic.ArraySortHelper`1[__Canon][System.__Canon]:Swap(ref,int,int)
          -4 : System.Private.CoreLib.dasm - System.Collections.Generic.GenericArraySortHelper`1[__Canon][System.__Canon]:SwapIfGreaterWithItems(ref,int,int)
          -4 : System.Private.CoreLib.dasm - System.Collections.Generic.GenericArraySortHelper`1[__Canon][System.__Canon]:Swap(ref,int,int)
22 total methods with size differences (8 improved, 14 regressed), 142893 unchanged.

[briansull]

Looks Good