spirv/nir: keep track of SPV_KHR_float_controls execution modes

v2:
- Add support for rounding modes for each floating point bit size.

v3:
- Commit e68871f6a4 ("spirv: Handle constants and types before
  execution modes") changed when the execution modes are handled,
  which affects the result of the floating point constants when the
  rounding mode is set in the execution mode. Moved the handling of
  the rounding modes before we handle the constants.

v4:
- Rename vtn_decoration "literals" to "operands" (Andres).
- Simplify execution mode parsing util function (Caio).
- Extend the comment about the timing of the handling of the rounding
  modes (Caio).

v5:
- Correct extension name (Caio).
- Rename shader info member (Andres).
- Rename float controls enum (Andres).

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com> [v3]
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>

src/compiler/shader_info.h

@@ -154,6 +154,9 @@ typedef struct shader_info {
    /** Was this shader linked with any transform feedback varyings? */
    bool has_transform_feedback_varyings;
 
+   /* SPV_KHR_float_controls: execution mode for floating point ops */
+   unsigned float_controls_execution_mode;
+
    union {
       struct {
          /* Which inputs are doubles */