Revert "avoid phi node for pointers flowing into Vec appends #130998"

This reverts PR <#130998> because
the added test seems to be flaky / non-deterministic, and has been
failing in unrelated PRs during merge CI.

Author: jieyouxu

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -517,16 +517,7 @@ pub(crate) fn llfn_attrs_from_instance<'ll, 'tcx>(
         to_add.push(llvm::CreateAllocKindAttr(cx.llcx, AllocKindFlags::Free));
         // applies to argument place instead of function place
         let allocated_pointer = AttributeKind::AllocatedPointer.create_attr(cx.llcx);
-        let attrs: &[_] = if llvm_util::get_version() >= (21, 0, 0) {
-            // "Does not capture provenance" means "if the function call stashes the pointer somewhere,
-            // accessing that pointer after the function returns is UB". That is definitely the case here since
-            // freeing will destroy the provenance.
-            let captures_addr = AttributeKind::CapturesAddress.create_attr(cx.llcx);
-            &[allocated_pointer, captures_addr]
-        } else {
-            &[allocated_pointer]
-        };
-        attributes::apply_to_llfn(llfn, AttributePlace::Argument(0), attrs);
+        attributes::apply_to_llfn(llfn, AttributePlace::Argument(0), &[allocated_pointer]);
     }
     if let Some(align) = codegen_fn_attrs.alignment {
         llvm::set_alignment(llfn, align);

library/alloc/src/slice.rs

@@ -448,11 +448,9 @@ impl<T> [T] {
                 // SAFETY:
                 // allocated above with the capacity of `s`, and initialize to `s.len()` in
                 // ptr::copy_to_non_overlapping below.
-                if s.len() > 0 {
-                    unsafe {
-                        s.as_ptr().copy_to_nonoverlapping(v.as_mut_ptr(), s.len());
-                        v.set_len(s.len());
-                    }
+                unsafe {
+                    s.as_ptr().copy_to_nonoverlapping(v.as_mut_ptr(), s.len());
+                    v.set_len(s.len());
                 }
                 v
             }

library/alloc/src/vec/mod.rs

@@ -2818,11 +2818,7 @@ impl<T, A: Allocator> Vec<T, A> {
         let count = other.len();
         self.reserve(count);
         let len = self.len();
-        if count > 0 {
-            unsafe {
-                ptr::copy_nonoverlapping(other as *const T, self.as_mut_ptr().add(len), count)
-            };
-        }
+        unsafe { ptr::copy_nonoverlapping(other as *const T, self.as_mut_ptr().add(len), count) };
         self.len += count;
     }