llvmpipe, draw: increase shader cache limits

llvmpipe, draw: increase shader cache limits

We're not particularly concerned with memory usage, if the tradeoff is
shader recompiles. And it's common for apps to have a lot of shaders
nowadays (and, since our shaders include a LOT of context state of course
we may create quite a bit more shaders even).
So quadruple the amount of shaders draw will cache (from 128 to 512).
For llvmpipe (fs shaders) quadruple the number of instructions, keep the
number of variants the same for now (only with very simple, non-texturing
shaders the variant limit could really be reached), and simplify the
definition, it's probably easier to just have one different definition
per branch...

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>