nouveau: Enable the NIR backend by default.

The glsl-to-tgsi code generation and GLSL IR linker is is going away
(!8044), so we need to make the call on whether to use nir-to-tgsi (See
!15932 and !15541), or switch over to the NIR code generator.  The NIR
backend should reduce the compile time regression while providing more
direct control over the IR we receive than going through NTT, while still
providing the optimization that NIR-to-TGSI was bringing us.

nv92 shader-db:
total local in shared programs: 3392 -> 1988 (-41.39%)
local in affected programs: 3392 -> 1988 (-41.39%)
total gpr in shared programs: 699567 -> 735407 (5.12%)
gpr in affected programs: 446658 -> 482498 (8.02%)
total instructions in shared programs: 6262828 -> 6052209 (-3.36%)
instructions in affected programs: 5184891 -> 4974272 (-4.06%)
total bytes in shared programs: 43380752 -> 41671128 (-3.94%)
bytes in affected programs: 37302616 -> 35592992 (-4.58%)
LOST:   0
GAINED: 1 (pixmark-piano)

nv120 shader-db:
total local in shared programs: 4416 -> 1988 (-54.98%)
local in affected programs: 4416 -> 1988 (-54.98%)
total gpr in shared programs: 870534 -> 893490 (2.64%)
gpr in affected programs: 564210 -> 587166 (4.07%)
total instructions in shared programs: 6379402 -> 6243210 (-2.13%)
instructions in affected programs: 5430790 -> 5294598 (-2.51%)
total bytes in shared programs: 68184224 -> 66729672 (-2.13%)
bytes in affected programs: 58013544 -> 56558992 (-2.51%)

This MR pulls in !15839 (closed) !15597 (merged) !14386 (merged) !14326 (closed) to fix NIR-backend regressions. Note that the NIR backend also resolves the WARNING: value %89 not uniquely defined family of warnings that the TGSI backend throws during shader-db with or without nir-to-tgsi.