Rollup merge of #92425 - calebzulawski:simd-cast, r=workingjubilee

Improve SIMD casts

* Allows `simd_cast` intrinsic to take `usize` and `isize`
* Adds `simd_as` intrinsic, which is the same as `simd_cast` except for saturating float-to-int conversions (matching the behavior of `as`).

cc `@workingjubilee`

compiler/rustc_span/src/symbol.rs

@@ -1223,6 +1223,7 @@ symbols! {
         simd,
         simd_add,
         simd_and,
+        simd_as,
         simd_bitmask,
         simd_cast,
         simd_ceil,