nir: Add ACCESS_CAN_SPECULATE

nir: Add ACCESS_CAN_SPECULATE

Determining whether it is safe to hoist a load instruction out of control flow
depends on complex hardware and driver details. Rather than encoding this as
knobs in every NIR pass that wants to do so (notably nir_opt_preamble and
nir_opt_peephole_select), add a per-load ACCESS flag for backends to set.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24011>