nir: Add algebraic optimizations for exponential/logarithmic functions.

nir: Add algebraic optimizations for exponential/logarithmic functions.

Most of these exist in the GLSL IR algebraic pass already.  However,
SSA allows us to find more instances of the patterns.

total NIR instructions in shared programs: 2015593 -> 2011430 (-0.21%)
NIR instructions in affected programs:     124189 -> 120026 (-3.35%)
helped:                                    604

total i965 instructions in shared programs: 6025505 -> 6018717 (-0.11%)
i965 instructions in affected programs:     261295 -> 254507 (-2.60%)
helped:                                     1295
HURT:                                       3

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>