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/meta/src/shared/instructions.rs

@@ -3280,6 +3280,9 @@ pub(crate) fn define(
         The input and output types must be storable to memory and of the same
         size. A bitcast is equivalent to storing one type and loading the other
         type from the same address.
+
+        For vector types, the lane types must also be the same size (see
+        `raw_bitcast` for changing the lane size).
         "#,
             &formats.unary,
         )