v2: Add some comments explaining some of the nuance of the shift
optimizations. Fix a bug in the shift count calculation of the upper
32-bits. Move the @64 from the variable to the opcode. All suggested
by Jordan.

No shader-db changes on any Intel platform.

fossil-db:

Meteor Lake and DG2 had similar results. (Meteor Lake shown)
Totals:
Instrs: 154507026 -> 154506576 (-0.00%)
Cycle count: 17436298868 -> 17436295016 (-0.00%)
Max live registers: 32635309 -> 32635297 (-0.00%)

Totals from 42 (0.01% of 632575) affected shaders:
Instrs: 5616 -> 5166 (-8.01%)
Cycle count: 133680 -> 129828 (-2.88%)
Max live registers: 1158 -> 1146 (-1.04%)

No fossil-db changes on any other Intel platform.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Part-of: <mesa/mesa!29148>