glsl: Add isinf() and isnan() builtins.

glsl: Add isinf() and isnan() builtins.

The implementations are as follows:

isinf(x) = (abs(x) == +infinity)
isnan(x) = (x != x)

Note: the latter formula is not necessarily obvious.  It works because
NaN is the only floating point number that does not equal itself.

Fixes piglit tests "isinf-and-isnan fs_basic" and "isinf-and-isnan
vs_basic".