Cranelift: fix regalloc2 integration bug wrt blockparam branch args. (#4042)

Previously, the block successor accumulation and the blockparam branch
arg setup were decoupled. The lowering backend implicitly specified
the order of successor edges via its `MachTerminator` enum on the last
instruction in the block, while the `Lower` toplevel
machine-independent driver set up blockparam branch args in the edge
order seen in CLIF.

In some cases, these orders did not match -- for example, when the
conditional branch depended on an FP condition that was implemented by
swapping taken/not-taken edges and inverting the condition code.

This PR refactors the successor handling to be centralized in `Lower`
rather than flow through the terminator `MachInst`, and adds a
successor block and its blockparam args at the same time, ensuring the
orders match.

cranelift/codegen/src/isa/x64/inst/mod.rs

@@ -2119,18 +2119,13 @@ impl MachInst for Inst {
         }
     }
 
-    fn is_term<'a>(&'a self) -> MachTerminator<'a> {
+    fn is_term(&self) -> MachTerminator {
         match self {
             // Interesting cases.
             &Self::Ret { .. } | &Self::EpiloguePlaceholder => MachTerminator::Ret,
-            &Self::JmpKnown { dst } => MachTerminator::Uncond(dst),
-            &Self::JmpCond {
-                taken, not_taken, ..
-            } => MachTerminator::Cond(taken, not_taken),
-            &Self::JmpTableSeq {
-                ref targets_for_term,
-                ..
-            } => MachTerminator::Indirect(&targets_for_term[..]),
+            &Self::JmpKnown { .. } => MachTerminator::Uncond,
+            &Self::JmpCond { .. } => MachTerminator::Cond,
+            &Self::JmpTableSeq { .. } => MachTerminator::Indirect,
             // All other cases are boring.
             _ => MachTerminator::None,
         }