glsl: Conditionalize atan2 math.

glsl: Conditionalize atan2 math.

In the old hand-writen implementation of atan2, the calculation of
atan(y/x) was performed conditionally in the "then" block of the
outermost if statement.  I believe I accidentally lifted this out
into unconditional code when converting to IR builder.

For reference, the original hand-written IR is visible in commit
722eff674b832e2321f791c68358ef52d2a1ff25.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: Erik Faye-Lund <kusmabite@gmail.com>