nir: fix denorms in unpack_half_1x16()

nir: fix denorms in unpack_half_1x16()

According to VK_KHR_shader_float_controls:

"Denormalized values obtained via unpacking an integer into a vector
 of values with smaller bit width and interpreting those values as
 floating-point numbers must: be flushed to zero, unless the entry
 point is declared with the code:DenormPreserve execution mode."

v2:
- Add nir_op_unpack_half_2x16_flush_to_zero opcode (Connor).

v3:
- Adapt to use the new NIR lowering framework (Andres).

v4:
- Updated to renamed shader info member and enum values (Andres).

v5:
- Simplify flags logic operations (Caio).

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com> [v2]