riscv64: Implement fcmp in ISLE (#5512)

Rework the compilation of fcmp in the riscv64 backend to be in ISLE, removing the need for the dedicated Fcmp instruction. This change is motivated by #5500, which showed that the riscv64 backend was generating branch instructions in the middle of a basic block.

We can't remove lower_br_fcmp quite yet as it's used in a few places in the emit module, but it's now no longer reachable from the ISLE lowerings.

Fixes #5500

cranelift/codegen/src/isa/riscv64/lower.isle

@@ -752,18 +752,10 @@
   (lower (icmp cc x @ (value_type ty) y))
   (lower_icmp cc x y ty))
 
-(decl gen_fcmp (FloatCC Value Value Type) Reg)
-(rule
-  (gen_fcmp cc x y ty)
-  (let
-    ((result WritableReg (temp_writable_reg $I64))
-      (_ Unit (emit (MInst.Fcmp cc result x y ty))))
-    (writable_reg_to_reg result)))
-
 ;;;;;  Rules for `fcmp`;;;;;;;;;
 (rule
   (lower (fcmp cc x @ (value_type ty) y))
-  (gen_fcmp cc x y ty))
+  (cmp_value (emit_fcmp cc ty x y)))
 
 ;;;;;  Rules for `func_addr`;;;;;;;;;
 (rule