nir/loop_analyze: Add a function to evaluate an ALU as constant

nir/loop_analyze: Add a function to evaluate an ALU as constant

...with a substitution. This function is largely a copy-and-paste of
try_fold_alu (nir_opt_constant_folding.c), and an argument could be made
that this function belongs in that file.

v2: Some changes were mistakenly squashed in to "nir/loop_analyze: Use
try_eval_const_alu and induction variable basis info" that should have
been here.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3445>