SM20 was smart enough to reduce shift immediates instead of just
detecting i20 overflow and adding copies. This adds helpers to make
this easier and propagates the improvement out to all the back-ends.
Even though it isn't necessary on Volta+, we might as well do it there
for consistency and because smaller shift values are easier to read in
the final assembly.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34678>
Faith Ekstrand
9890110856