Win64 ABI: Pass/return delegates like slices - in (up to) 2 GP registers

I expect this to be slightly more performant than the previous behavior,
where a delegate was treated like a corresponding struct, passed via
hidden pointer and returned via sret.

The primary motivation is a smooth preparation for PR #3578 - in order
to allow people to experiment with `-preview=in` without recompiling
druntime and Phobos, `in` slices and delegates must not be passed by-ref
with `-preview=in` (see dlang/dmd#11828). This would have required a
special case for delegates on Win64, which is IMO better handled this
way.

Author: kinke

gen/abi-win64.cpp

@@ -71,7 +71,11 @@ struct Win64TargetABI : TargetABI {
 
     // Remaining aggregates which can NOT be rewritten as integers (size > 8
     // bytes or not a power of 2) are passed by ref to hidden copy.
-    return isAggregate(t) && !canRewriteAsInt(t);
+    // LDC-specific exceptions: slices and delegates are left alone (as non-
+    // rewritten IR structs) and passed/returned as 2 separate args => passed in
+    // up to 2 GP registers and returned in RAX & RDX.
+    return isAggregate(t) && !canRewriteAsInt(t) && t->ty != Tarray &&
+           t->ty != Tdelegate;
   }
 
 public:
@@ -113,6 +117,7 @@ struct Win64TargetABI : TargetABI {
     //   are returned in a register (RAX, or XMM0 for single float/ifloat/
     //   double/idouble)
     // * 80-bit real/ireal are returned on the x87 stack
+    // * LDC-specific: slices and delegates are returned in RAX & RDX
     // * all other types are returned via sret
     return passPointerToHiddenCopy(rt, /*isReturnValue=*/true, tf->linkage);
   }