glsl: Add opcodes for atan and atan2

glsl: Add opcodes for atan and atan2

Adds ir_binop_atan2 and ir_unop_atan. When converting to NIR these are
expanded out using the appropriate builtin generator. If they are used
with anything else then it will just hit an assert.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>