nir: make nir_fisnan helper exact
Floating point ALU assume no NaNs unless FLOAT_CONTROLS_SIGNED_ZERO_INF_NAN_PRESERVE_FPn or (for some opcodes) exact=true.
Floating point ALU assume no NaNs unless FLOAT_CONTROLS_SIGNED_ZERO_INF_NAN_PRESERVE_FPn or (for some opcodes) exact=true.