const folding: select

Author: Firestar99

crates/rustc_codegen_spirv/src/builder/builder_methods.rs

@@ -3071,6 +3071,14 @@ impl<'a, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'tcx> {
     ) -> Self::Value {
         assert_ty_eq!(self, then_val.ty, else_val.ty);
         let result_type = then_val.ty;
+
+        if let Some(const_cond) = self.try_get_const_int_value(cond) {
+            if let ConstValue::Bool(b) = const_cond {
+                // as we directly return the values, it'll preserve their constness as well
+                return if b { then_val } else { else_val };
+            }
+        }
+
         self.emit()
             .select(
                 result_type,