nir/lower_double_ops: lower trunc()

At least i965 hardware does not have native support for truncating doubles.

v2:
  - Simplified the implementation significantly.
  - Fixed the else branch, that was not doing what we wanted.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir.h

@@ -2417,6 +2417,7 @@ typedef enum {
    nir_lower_drcp = (1 << 0),
    nir_lower_dsqrt = (1 << 1),
    nir_lower_drsq = (1 << 2),
+   nir_lower_dtrunc = (1 << 3),
 } nir_lower_doubles_options;
 
 void nir_lower_doubles(nir_shader *shader, nir_lower_doubles_options options);