llvmpipe: increase fs shader variant instruction cache limit by factor 4

llvmpipe: increase fs shader variant instruction cache limit by factor 4

The previous limit of of 128*1024 was reported to cause frequent recompiles
in some apps due to shader variant thrashing on IRC in some apps leading
to noticeable lags.
Note that the LP_MAX_SHADER_VARIANTS limit (1024) was more or less impossible
to reach, since even simple fragment shaders without texturing (glxgears) used
more than twice than 128 instructions, hence the instruction limit would have
always been reached first (excluding things like trivial shaders not writing
color). Even with the new limit it is VERY likely the instruction limit is hit
first.
Should help with such lags due to recompiles (though other shader types have
their own limits, LP_MAX_SETUP_VARIANTS and DRAW_MAX_SHADER_VARIANTS, in
particular the latter seems a bit small (128)).

Reviewed-by: Brian Paul <brianp@vmware.com>