Port vconst to ISLE (AArch64) (#4750)

* Port `vconst` to ISLE (AArch64)

Ported the existing implementation of `vconst` to ISLE for AArch64, and
added support for 64-bit vector constants.

Also introduced 64-bit `vconst` support to the interpreter.

Copyright (c) 2022 Arm Limited

* Replace if-chains with match statements

Copyright (c) 2022 Arm Limited

cranelift/codegen/src/isa/aarch64/inst.isle

@@ -2435,6 +2435,11 @@
       (if-let addr_reg (amode_is_reg addr))
       addr_reg)
 
+;; Lower a constant f64.
+(decl constant_f64 (u64) Reg)
+;; TODO: Port lower_constant_f64() to ISLE.
+(extern constructor constant_f64 constant_f64)
+
 ;; Lower a constant f128.
 (decl constant_f128 (u128) Reg)
 ;; TODO: Port lower_constant_f128() to ISLE.