i8x16.popcnt instruction (#379)

Author: Maratyszcza

proposals/simd/BinarySIMD.md

@@ -258,3 +258,4 @@ For example, `ImmLaneIdx16` is a byte with values in the range 0-15 (inclusive).
 | `i32x4.trunc_sat_f64x2_u_zero` |     `TBD`| -                        |
 | `f32x4.demote_f64x2_zero`      |     `TBD`| -                        |
 | `f64x2.promote_low_f32x4`      |     `TBD`| -                        |
+| `i8x16.popcnt`                 |     `TBD`| -                        |

proposals/simd/ImplementationStatus.md

@@ -226,6 +226,7 @@
 | `i32x4.trunc_sat_f64x2_u_zero` |                           |                    |                    |                    |                    |
 | `f32x4.demote_f64x2_zero`      |                           |                    |                    |                    |                    |
 | `f64x2.promote_low_f32x4`      |                           |                    |                    |                    |                    |
+| `i8x16.popcnt`              |                           |                    |                    |                    |                    |
 
 [1] Tip of tree LLVM as of May 20, 2020
 

proposals/simd/SIMD.md

@@ -662,6 +662,15 @@ Note that the normal WebAssembly `select` instruction also works with vector
 types. It selects between two whole vectors controlled by a single scalar value,
 rather than selecting bits controlled by a control mask vector.
 
+### Lane-wise Population Count
+* `i8x16.popcnt(v: v128) -> v128`
+
+Count the number of bits set to one within each lane.
+
+```python
+def S.popcnt(v):
+    return S.lanewise_unary(popcnt, v)
+```
 
 ## Boolean horizontal reductions