Implemented the following Opcodes for the Cranelift interpreter:
- `Unarrow` to combine two SIMD vectors into a new vector with twice
the lanes but half the width, with signed inputs which are clamped to
`0x00`.
- `Uunarrow` to perform the same operation as `Unarrow` but treating
inputs as unsigned.
- `Snarrow` to perform the same operation as `Unarrow` but treating
both inputs and outputs as signed, and saturating accordingly.
Note that all 3 instructions saturate at the type boundaries.
Copyright (c) 2021, Arm Limited
83c3bc5b9d