agx: Optimize multiplies

agx: Optimize multiplies

We have an imad instruction and our iadd has a small immediate shift on the
second source. Together, these allow expressing lots of integer multiplies more
efficiently. Add some rules to optimize these now that the backend compiler can
ingest the optimized forms.

Half-register changes are from load_const scheduling changing in some vertex
shaders.

   total instructions in shared programs: 1539092 -> 1537949 (-0.07%)
   instructions in affected programs: 167896 -> 166753 (-0.68%)

   total bytes in shared programs: 10543012 -> 10533866 (-0.09%)
   bytes in affected programs: 1218068 -> 1208922 (-0.75%)

   total halfregs in shared programs: 483180 -> 483448 (0.06%)
   halfregs in affected programs: 1942 -> 2210 (13.80%)

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22695>