Rollup merge of rust-lang#137953 - RalfJung:simd-intrinsic-masks, r=WaffleLapkin

simd intrinsics with mask: accept unsigned integer masks, and fix some of the errors

It's not clear at all why the mask would have to be signed, it is anyway interpreted bitwise. The backend should just make sure that works no matter the surface-level type; our LLVM backend already does this correctly. The note of "the mask may be widened, which only has the correct behavior for signed integers" explains... nothing? Why can't the code do the widening correctly? If necessary, just cast to the signed type first...

Also while we are at it, fix the errors. For simd_masked_load/store, the errors talked about the "third argument" but they meant the first argument (the mask is the first argument there). They also used the wrong type for `expected_element`.

I have extremely low confidence in the GCC part of this PR.

See [discussion on Zulip](https://rust-lang.zulipchat.com/#narrow/channel/257879-project-portable-simd/topic/On.20the.20sign.20of.20masks)

Author: ChrisDenton

core/src/intrinsics/simd.rs

@@ -304,7 +304,7 @@ pub unsafe fn simd_shuffle<T, U, V>(x: T, y: T, idx: U) -> V;
 ///
 /// `U` must be a vector of pointers to the element type of `T`, with the same length as `T`.
 ///
-/// `V` must be a vector of signed integers with the same length as `T` (but any element size).
+/// `V` must be a vector of integers with the same length as `T` (but any element size).
 ///
 /// For each pointer in `ptr`, if the corresponding value in `mask` is `!0`, read the pointer.
 /// Otherwise if the corresponding value in `mask` is `0`, return the corresponding value from
@@ -325,7 +325,7 @@ pub unsafe fn simd_gather<T, U, V>(val: T, ptr: U, mask: V) -> T;
 ///
 /// `U` must be a vector of pointers to the element type of `T`, with the same length as `T`.
 ///
-/// `V` must be a vector of signed integers with the same length as `T` (but any element size).
+/// `V` must be a vector of integers with the same length as `T` (but any element size).
 ///
 /// For each pointer in `ptr`, if the corresponding value in `mask` is `!0`, write the
 /// corresponding value in `val` to the pointer.
@@ -349,7 +349,7 @@ pub unsafe fn simd_scatter<T, U, V>(val: T, ptr: U, mask: V);
 ///
 /// `U` must be a pointer to the element type of `T`
 ///
-/// `V` must be a vector of signed integers with the same length as `T` (but any element size).
+/// `V` must be a vector of integers with the same length as `T` (but any element size).
 ///
 /// For each element, if the corresponding value in `mask` is `!0`, read the corresponding
 /// pointer offset from `ptr`.
@@ -372,7 +372,7 @@ pub unsafe fn simd_masked_load<V, U, T>(mask: V, ptr: U, val: T) -> T;
 ///
 /// `U` must be a pointer to the element type of `T`
 ///
-/// `V` must be a vector of signed integers with the same length as `T` (but any element size).
+/// `V` must be a vector of integers with the same length as `T` (but any element size).
 ///
 /// For each element, if the corresponding value in `mask` is `!0`, write the corresponding
 /// value in `val` to the pointer offset from `ptr`.
@@ -556,7 +556,7 @@ pub unsafe fn simd_bitmask<T, U>(x: T) -> U;
 ///
 /// `T` must be a vector.
 ///
-/// `M` must be a signed integer vector with the same length as `T` (but any element size).
+/// `M` must be an integer vector with the same length as `T` (but any element size).
 ///
 /// For each element, if the corresponding value in `mask` is `!0`, select the element from
 /// `if_true`.  If the corresponding value in `mask` is `0`, select the element from