spv-in: Fix incorrect translation of SMod (#1995)

Since spirv's SMod doesn't map to naga's IR modulo operator the
instruction is mapped into it's direct formula, this formula requires
some casts from int to float and back.

These casts were present but their `convert` field was left to `None`
which implied that they were bitcasts, causing some wildly incorrect
results.

This commit fixes it by setting the `convert` field to the same size as
the result type.

src/front/spv/mod.rs

@@ -2045,19 +2045,24 @@ impl<I: Iterator<Item = u32>> Parser<I> {
                         body_idx,
                     );
 
+                    let result_ty = self.lookup_type.lookup(result_type_id)?;
+                    let inner = &ctx.type_arena[result_ty.handle].inner;
+                    let kind = inner.scalar_kind().unwrap();
+                    let size = inner.size(ctx.const_arena) as u8;
+
                     let left_cast = ctx.expressions.append(
                         crate::Expression::As {
                             expr: left,
                             kind: crate::ScalarKind::Float,
-                            convert: None,
+                            convert: Some(size),
                         },
                         span,
                     );
                     let right_cast = ctx.expressions.append(
                         crate::Expression::As {
                             expr: right,
                             kind: crate::ScalarKind::Float,
-                            convert: None,
+                            convert: Some(size),
                         },
                         span,
                     );
@@ -2079,16 +2084,11 @@ impl<I: Iterator<Item = u32>> Parser<I> {
                         },
                         span,
                     );
-                    let result_ty = self.lookup_type.lookup(result_type_id)?;
-                    let kind = ctx.type_arena[result_ty.handle]
-                        .inner
-                        .scalar_kind()
-                        .unwrap();
                     let cast = ctx.expressions.append(
                         crate::Expression::As {
                             expr: floor,
                             kind,
-                            convert: None,
+                            convert: Some(size),
                         },
                         span,
                     );