Revert use of Mesa IR optimizer for ARB_fragment_programs

Revert use of Mesa IR optimizer for ARB_fragment_programs

Commit f82f2fb3dc770902f1657ab1c22e6004faa3afab added use of the Mesa
IR optimizer for both ARB_fragment_program and ARB_vertex_program, but
only justified the vertex-program portions with measured performance
improvements.

Meanwhile, the optimizer was seen to generate hundreds of unused
immediates without discarding them, causing failures.

Discard the use of the optimizer for now to fix the regression. (In
the future, we anticpate things moving from Mesa IR to NIR for better
optimization anyway.)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82477

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
CC: "10.3 10.4 10.5" <mesa-stable@lists.freedesktop.org>