Rollup merge of #131664 - taiki-e:s390x-asm-vreg-inout, r=Amanieu

Support input/output in vector registers of s390x inline assembly (under asm_experimental_reg feature)

This extends currently clobber-only vector registers (`vreg`) support to allow passing `#[repr(simd)]` types, floats (f32/f64/f128), and integers (i32/i64/i128) as input/output.

This is unstable and gated under new `#![feature(asm_experimental_reg)]` (tracking issue: rust-lang/rust#133416). If the feature is not enabled, only clober is supported as before.

| Architecture | Register class | Target feature | Allowed types |
| ------------ | -------------- | -------------- | -------------- |
| s390x | `vreg` | `vector` | `i32`, `f32`, `i64`, `f64`, `i128`, `f128`, `i8x16`, `i16x8`, `i32x4`, `i64x2`, `f32x4`, `f64x2` |

This matches the list of types that are supported by the vector registers in LLVM:
/~https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/SystemZ/SystemZRegisterInfo.td#L301-L313

In addition to `core::simd` types and floats listed above, custom `#[repr(simd)]` types of the same size and type are also allowed. All allowed types other than i32/f32/i64/f64/i128, and relevant target features are currently unstable.

Currently there is no SIMD type for s390x in `core::arch`, but this is tracked in rust-lang/rust#130869.

cc rust-lang/rust#130869 about vector facility support in s390x
cc rust-lang/rust#125398 & rust-lang/rust#116909 about f128 support in asm

`@rustbot` label +O-SystemZ +A-inline-assembly

src/inline_asm.rs

@@ -462,8 +462,12 @@ impl<'tcx> InlineAssemblyGenerator<'_, 'tcx> {
         let mut slots_output = vec![None; self.operands.len()];
 
         let new_slot_fn = |slot_size: &mut Size, reg_class: InlineAsmRegClass| {
-            let reg_size =
-                reg_class.supported_types(self.arch).iter().map(|(ty, _)| ty.size()).max().unwrap();
+            let reg_size = reg_class
+                .supported_types(self.arch, true)
+                .iter()
+                .map(|(ty, _)| ty.size())
+                .max()
+                .unwrap();
             let align = rustc_abi::Align::from_bytes(reg_size.bytes()).unwrap();
             let offset = slot_size.align_to(align);
             *slot_size = offset + reg_size;