Rollup merge of #131551 - taiki-e:ppc-asm-vreg-inout, r=Amanieu

Support input/output in vector registers of PowerPC inline assembly

This extends currently clobber-only vector registers (`vreg`) support to allow passing `#[repr(simd)]` types as input/output.

| Architecture | Register class | Target feature | Allowed types |
| ------------ | -------------- | -------------- | -------------- |
| PowerPC      | `vreg` | `altivec` | `i8x16`, `i16x8`, `i32x4`, `f32x4` |
| PowerPC      | `vreg` | `vsx` | `f32`, `f64`, `i64x2`, `f64x2` |

In addition to floats and `core::simd` types listed above, `core::arch` types and custom `#[repr(simd)]` types of the same size and type are also allowed. All allowed types and relevant target features are currently unstable.

r? `@Amanieu`

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

compiler/rustc_target/src/asm/powerpc.rs

@@ -51,7 +51,11 @@ impl PowerPCInlineAsmRegClass {
                 }
             }
             Self::freg => types! { _: F32, F64; },
-            Self::vreg => &[],
+            // FIXME: vsx also supports integers?: https://github.com/rust-lang/rust/pull/131551#discussion_r1862535963
+            Self::vreg => types! {
+                altivec: VecI8(16), VecI16(8), VecI32(4), VecF32(4);
+                vsx: F32, F64, VecI64(2), VecF64(2);
+            },
             Self::cr | Self::xer => &[],
         }
     }