This optimizes evaluation of chained indexing operations by first
looking for an lvalue 'root' node and copying from it directly into the
topmost rvalue, instead of creating potentially many temporaries.

Change-Id: I47cf9b0230bd305695e7b2a44ddc44bc1320b764
Reviewed-on: https://swiftshader-review.googlesource.com/13908Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>