nir/lower_double_ops: lower floor()

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

v2 (Sam):
  - Improve the lowering pass to remove one bcsel (Jason)

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir.h

@@ -2418,6 +2418,7 @@ typedef enum {
    nir_lower_dsqrt = (1 << 1),
    nir_lower_drsq = (1 << 2),
    nir_lower_dtrunc = (1 << 3),
+   nir_lower_dfloor = (1 << 4),
 } nir_lower_doubles_options;
 
 void nir_lower_doubles(nir_shader *shader, nir_lower_doubles_options options);