nir/builder: Handle i2b conversions specially in nir_type_convert

nir/builder: Handle i2b conversions specially in nir_type_convert

The shaders affected here are ones that were previously affected when
i2b was unconditionally lowered in opt_algebraic. There are a few places
where some transformations happen in a different order, so some
algebraic patterns are missed.

All Broadwell and newer Intel platforms had similar results. (Ice Lake shown)
total instructions in shared programs: 19914369 -> 19914566 (<.01%)
instructions in affected programs: 92375 -> 92572 (0.21%)
helped: 0 / HURT: 90

total cycles in shared programs: 853851470 -> 853867215 (<.01%)
cycles in affected programs: 12400663 -> 12416408 (0.13%)
helped: 28 / HURT: 69

Haswell and Ivy Bridge had similar results. (Haswell shown)
total instructions in shared programs: 16710721 -> 16710700 (<.01%)
instructions in affected programs: 108010 -> 107989 (-0.02%)
helped: 57 / HURT: 103

total cycles in shared programs: 884299412 -> 884306546 (<.01%)
cycles in affected programs: 12986423 -> 12993557 (0.05%)
helped: 87 / HURT: 102

total spills in shared programs: 14937 -> 14925 (-0.08%)
spills in affected programs: 12 -> 0
helped: 9 / HURT: 0

total fills in shared programs: 17569 -> 17557 (-0.07%)
fills in affected programs: 12 -> 0
helped: 9 / HURT: 0

Sandy Bridge
total instructions in shared programs: 13902341 -> 13902347 (<.01%)
instructions in affected programs: 7311 -> 7317 (0.08%)
helped: 3 / HURT: 8

total cycles in shared programs: 741795500 -> 741792266 (<.01%)
cycles in affected programs: 273308 -> 270074 (-1.18%)
helped: 9 / HURT: 2

No shader-db changes on any other Intel platform.

No fossil-db changes on any Intel platform.

Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15121>