glsl/builtin: Add alternate versions of atan using new ops

glsl/builtin: Add alternate versions of atan using new ops

Adds alternate versions of the atan builtin functions that use
ir_unop_atan and ir_binop_atan2 instead of inlining to the IR
implementation of the function. These alternatives are selected if the
IR is going to be consumed by NIR. In that case the IR ops will be
translated to the appropriate NIR op.

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