cranelift: Add a conditional branch instruction with two targets (#5446)

Add a conditional branch instruction with two targets: brif. This instruction will eventually replace brz and brnz, as it encompasses the behavior of both.

This PR also changes the InstructionData layout for instruction formats that hold BlockCall values, taking the same approach we use for Value arguments. This allows branch_destination to return a slice to the BlockCall values held in the instruction, rather than requiring that we pattern match on InstructionData to fetch the then/else blocks.

Function generation for fuzzing has been updated to generate uses of brif, and I've run the cranelift-fuzzgen target locally for hours without triggering any new failures.

cranelift/codegen/src/isa/x64/lower.isle

@@ -2885,6 +2885,25 @@
 (rule (lower_branch (jump _) (single_target target))
       (emit_side_effect (jmp_known target)))
 
+;; Rules for `brif` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(rule 2 (lower_branch (brif (maybe_uextend (icmp cc a b)) _ _) (two_targets then else))
+        (emit_side_effect (jmp_cond_icmp (emit_cmp cc a b) then else)))
+
+(rule 2 (lower_branch (brif (maybe_uextend (fcmp cc a b)) _ _) (two_targets then else))
+        (emit_side_effect (jmp_cond_fcmp (emit_fcmp cc a b) then else)))
+
+(rule 1 (lower_branch (brif val @ (value_type $I128) _ _)
+                      (two_targets then else))
+      (emit_side_effect (jmp_cond_icmp (cmp_zero_i128 (CC.Z) val) then else)))
+
+(rule (lower_branch (brif val @ (value_type (ty_int_bool_or_ref)) _ _)
+                    (two_targets then else))
+      (emit_side_effect (with_flags_side_effect
+                          (cmp_zero_int_bool_ref val)
+                          (jmp_cond (CC.NZ) then else))))
+
+
 ;; Rules for `brz` and `brnz` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (rule 2 (lower_branch (brz (maybe_uextend (icmp cc a b)) _) (two_targets taken not_taken))

cranelift/codegen/src/isa/x64/lower/isle.rs

@@ -20,7 +20,7 @@ use crate::{
         condcodes::{CondCode, FloatCC, IntCC},
         immediates::*,
         types::*,
-        Inst, InstructionData, MemFlags, Opcode, TrapCode, Value, ValueList,
+        BlockCall, Inst, InstructionData, MemFlags, Opcode, TrapCode, Value, ValueList,
     },
     isa::{
         unwind::UnwindInst,