Only try folding constant array indexing if propagated.

Only try folding constant array indexing if propagated.

The array folding logic generated an error when a constant array didn't
have an array of literal constants.

Change-Id: I48f58936cfa7e19e0f4026fb940654f20d9a9d5e
Reviewed-on: https://swiftshader-review.googlesource.com/15551
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>