Implement Unarrow, Uunarrow, and Snarrow for the interpreter

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

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

@@ -4048,7 +4048,7 @@ pub(crate) fn define(
         Combine `x` and `y` into a vector with twice the lanes but half the integer width while
         saturating overflowing values to the unsigned maximum and minimum.
 
-        Note that all input lanes are considered unsigned.
+        Note that all input lanes are considered unsigned: any negative values will be interpreted as unsigned, overflowing and being replaced with the unsigned maximum.
 
         The lanes will be concatenated after narrowing. For example, when `x` and `y` are `i32x4`
         and `x = [x3, x2, x1, x0]` and `y = [y3, y2, y1, y0]`, then after narrowing the value