Rounding Average instructions (#126)

Author: Maratyszcza

proposals/simd/BinarySIMD.md

@@ -201,3 +201,5 @@ The `v8x16.shuffle` instruction has 16 bytes after `simdop`.
 | `i64x2.load32x2_s`         |    `0xd6`| m:memarg           |
 | `i64x2.load32x2_u`         |    `0xd7`| m:memarg           |
 | `v128.andnot`              |    `0xd8`| -                  |
+| `i8x16.avgr_u`             |    `0xd9`|                    |
+| `i16x8.avgr_u`             |    `0xda`|                    |

proposals/simd/ImplementationStatus.md

@@ -91,6 +91,7 @@
 | `i8x16.min_u`              | `-munimplemented-simd128` |    :heavy_check_mark: |                    |                    |
 | `i8x16.max_s`              | `-munimplemented-simd128` |    :heavy_check_mark: |                    |                    |
 | `i8x16.max_u`              | `-munimplemented-simd128` |    :heavy_check_mark: |                    |                    |
+| `i8x16.avgr_u`             |                           |                       |                    |                    |
 | `i16x8.neg`                |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `i16x8.any_true`           |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `i16x8.all_true`           |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
@@ -108,6 +109,7 @@
 | `i16x8.min_u`              | `-munimplemented-simd128` |    :heavy_check_mark: |                    |                    |
 | `i16x8.max_s`              | `-munimplemented-simd128` |    :heavy_check_mark: |                    |                    |
 | `i16x8.max_u`              | `-munimplemented-simd128` |    :heavy_check_mark: |                    |                    |
+| `i16x8.avgr_u`             |                           |                       |                    |                    |
 | `i32x4.neg`                |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `i32x4.any_true`           |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `i32x4.all_true`           |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |

proposals/simd/SIMD.md

@@ -488,6 +488,21 @@ each pair.
 def S.max(a, b):
     return S.lanewise_binary(max, a, b)
 ```
+
+### Lane-wise integer rounding average
+* `i8x16.avgr_u(a: v128, b: v128) -> v128`
+* `i16x8.avgr_u(a: v128, b: v128) -> v128`
+
+Lane-wise rounding average:
+
+```python
+def S.RoundingAverage(x, y):
+    return (x + y + 1) // 2
+
+def S.avgr_u(a, b):
+    return S.lanewise_binary(S.RoundingAverage, S.AsUnsigned(a), S.AsUnsigned(b))
+```
+
 ## Bit shifts
 
 ### Left shift by scalar