Support negative array ofsets in painless

Adds support for indexing into lists and arrays with negative
indexes meaning "counting from the back". So for if
`x = ["cat", "dog", "chicken"]` then `x[-1] == "chicken"`.

This adds an extra branch to every array and list access but
some performance testing makes it look like the branch predictor
successfully predicts the branch every time so there isn't a
in execution time for this feature when the index is positive.
When the index is negative performance testing showed the runtime
is the same as writing `x[x.length - 1]`, again, presumably thanks
to the branch predictor.

Those performance metrics were calculated for lists and arrays but
`def`s get roughly the same treatment though instead of inlining
the test they need to make a invoke dynamic so we don't screw up
maps.

Closes #20870

docs/reference/modules/scripting/painless-syntax.asciidoc

@@ -28,6 +28,23 @@ String constants can be declared with single quotes, to avoid escaping horrors w
 def mystring = 'foo';
 ---------------------------------------------------------
 
+[float]
+[[painless-arrays]]
+==== Arrays
+
+Arrays can be subscripted starting from `0` for traditional array access or with
+negative numbers to starting from the back of the array. So the following
+returns `2`.
+
+[source,painless]
+---------------------------------------------------------
+int[] x = new int[5];
+x[0]++;
+x[-5]++;
+return x[0];
+---------------------------------------------------------
+
+
 [float]
 [[painless-lists]]
 ==== List
@@ -39,11 +56,13 @@ Lists can be created explicitly (e.g. `new ArrayList()`) or initialized similar
 def list = [1,2,3];
 ---------------------------------------------------------
 
-Lists can also be accessed similar to arrays: they support subscript and `.length`:
+Lists can also be accessed similar to arrays. They support `.length` and
+subscripts, including negative subscripts to read from the back of the list:
 
 [source,painless]
 ---------------------------------------------------------
 def list = [1,2,3];
+list[-1] = 5
 return list[0]
 ---------------------------------------------------------