Merge raw_bitcast and bitcast (#5175)

- Allow bitcast for vectors with differing lane widths
- Remove raw_bitcast IR instruction
- Change all users of raw_bitcast to bitcast
- Implement support for no-op bitcast cases across backends

This implements the second step of the plan outlined here:
https://github.com/bytecodealliance/wasmtime/issues/4566#issuecomment-1234819394

cranelift/interpreter/src/step.rs

@@ -947,7 +947,7 @@ where
         Opcode::Nearest => assign(Value::nearest(arg(0)?)?),
         Opcode::IsNull => unimplemented!("IsNull"),
         Opcode::IsInvalid => unimplemented!("IsInvalid"),
-        Opcode::Bitcast | Opcode::RawBitcast | Opcode::ScalarToVector => {
+        Opcode::Bitcast | Opcode::ScalarToVector => {
             let input_ty = inst_context.type_of(inst_context.args()[0]).unwrap();
             let arg0 = extractlanes(&arg(0)?, input_ty)?;