turnip: Short-circuit if ladder generation for constant index SSBO/UBOs.

turnip: Short-circuit if ladder generation for constant index SSBO/UBOs.

The compiler *can* eventually chew through all the copy prop, constant
folding, and dead_cf necessary to use just our constant index, but we can
save a whole lot of hassle by chasing the MOVs up front and finding the
constant.

dEQP-VK.ubo.3_level_array.scalar.row_major_mat4.both goes from 2.0s to
1.6s on a release build (3.1s to 2.1s for a debug build like we use in CI).

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11613>