Vector bitcast support (AArch64 & Interpreter) (#4820)

* Vector bitcast support (AArch64 & Interpreter)

Implemented support for `bitcast` on vector values for AArch64 and the
interpreter.

Also corrected the verifier to ensure that the size, in bits, of the input and
output types match for a `bitcast`, per the docs.

Copyright (c) 2022 Arm Limited

* `I128` same-type bitcast support

Copyright (c) 2022 Arm Limited

* Directly return input for 64-bit GPR<=>GPR bitcast

Copyright (c) 2022 Arm Limited

cranelift/codegen/src/verifier/mod.rs

@@ -1096,15 +1096,26 @@ impl<'a> Verifier<'a> {
         let typ = self.func.dfg.ctrl_typevar(inst);
         let value_type = self.func.dfg.value_type(arg);
 
-        if typ.lane_bits() < value_type.lane_bits() {
+        if typ.lane_bits() != value_type.lane_bits() {
             errors.fatal((
                 inst,
                 format!(
-                    "The bitcast argument {} doesn't fit in a type of {} bits",
+                    "The bitcast argument {} has a lane type of {} bits, which doesn't match an expected type of {} bits",
                     arg,
+                    value_type.lane_bits(),
                     typ.lane_bits()
                 ),
             ))
+        } else if typ.bits() != value_type.bits() {
+            errors.fatal((
+                inst,
+                format!(
+                    "The bitcast argument {} has a type of {} bits, which doesn't match an expected type of {} bits",
+                    arg,
+                    value_type.bits(),
+                    typ.bits()
+                ),
+            ))
         } else {
             Ok(())
         }