This makes WalkAccessChain slightly easier on the backend -- there is
less constant folding to do.

Bug: b/124388146

Change-Id: I4e76c3e494278e1399b8a86134b652c2a6d96d8c
Reviewed-on: https://swiftshader-review.googlesource.com/c/24988Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>