Rollup merge of #157130 - alexcrichton:refactor-cast-target-internals, r=nnethercote

Use a `ArrayVec` in `CastTarget`

This commit switches a fixed-size list of `[Option<Reg>; 8]` to instead
holding `ArrayVec<Reg, 8>` in the `CastTarget` type used when
calculating ABIs. This is inspired by [discussion on Zulip][link] where
I'm hoping to in the near future extend the usage of this to possibly
beyond 8 elements for a new WebAssembly ABI taking advantage of
multi-value. For now though this mostly just switches to
array/slice-like idioms of accessors rather than dealing with
`Option<Reg>` as the unit.

[link]: https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Using.20.60ArgAbi.3A.3Amake_direct_deprecated.60/with/598607139

Author: JonathanBrouwer

src/abi.rs

@@ -46,7 +46,7 @@ impl GccType for CastTarget {
             )
         };
 
-        if self.prefix.iter().all(|x| x.is_none()) {
+        if self.prefix.is_empty() {
             // Simplify to a single unit when there is no prefix and size <= unit size
             if self.rest.total <= self.rest.unit.size {
                 return rest_gcc_unit;
@@ -62,7 +62,7 @@ impl GccType for CastTarget {
         let mut args: Vec<_> = self
             .prefix
             .iter()
-            .flat_map(|option_reg| option_reg.map(|reg| reg.gcc_type(cx)))
+            .map(|reg| reg.gcc_type(cx))
             .chain((0..rest_count).map(|_| rest_gcc_unit))
             .collect();