Port AvgRound & SqmulRoundSat to ISLE (AArch64) (#4639)

Ported the existing implementations of the following opcodes on AArch64
to ISLE:
- `AvgRound`
  - Also introduced support for `i64x2` vectors, as per the docs.
- `SqmulRoundSat`

Copyright (c) 2022 Arm Limited

cranelift/codegen/meta/src/shared/instructions.rs

@@ -592,6 +592,8 @@ fn define_simd_arithmetic(
             "avg_round",
             r#"
         Unsigned average with rounding: `a := (x + y + 1) // 2`
+
+        The addition does not lose any information (such as from overflow).
         "#,
             &formats.binary,
         )