r600g/sb: improve math optimizations v2

r600g/sb: improve math optimizations v2

This patch adds support for some math optimizations that are generally
considered unsafe, that's why they are currently disabled for compute
shaders.

GL requirements are less strict, so they are enabled for
for GL shaders by default. In case of any issues with
applications that rely on higher precision than guaranteed by GL,
'sbsafemath' option in R600_DEBUG allows to disable them.

v2 - always set proper src vector size for transformed instructions
   - check for clamp modifier in the expr_handler::fold_assoc

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>