glsl: Use the generated constant expression code

glsl: Use the generated constant expression code

Immediately previous to this patch,

    diff -wud src/glsl/ir_constant_expression.cpp \
              src/glsl/ir_expression_operation_constant.h

should be "minimal."

v3: With much help from José Fonseca, fix the SCons build.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>