Update all uses of variadic `.Cases` to use the initializer list
overload instead. I plan to mark variadic `.Cases` as deprecated in a
followup PR.

For more context, see https://github.com/llvm/llvm-project/pull/163117.

Generate a usage error instead of asserting.

Module flag is used to indicate the feature to be propagated to the
function. As now the frontend emits all attributes accordingly let's
help the auto upgrade to only do work when old and new bitcodes are
merged.

Depends on #82819 and #86031

The `masked.load`, `masked.store`, `masked.gather` and `masked.scatter`
intrinsics currently accept a separate alignment immarg. Replace this
with an `align` attribute on the pointer / vector of pointers argument.

This is the standard representation for alignment information on
intrinsics, and is already used by all other memory intrinsics. This
means the signatures now match llvm.expandload, llvm.vp.load, etc.
(Things like llvm.memcpy used to have a separate alignment argument as
well, but were already migrated a long time ago.)

It's worth noting that the masked.gather and masked.scatter intrinsics
previously accepted a zero alignment to indicate the ABI type alignment
of the element type. This special case is gone now: If the align
attribute is omitted, the implied alignment is 1, as usual. If ABI
alignment is desired, it needs to be explicitly emitted (which the
IRBuilder API already requires anyway).

Summary:
The changes in https://github.com/llvm/llvm-project/pull/163011 caused
all ELF platforms to default to ELF mangling. We want to auto upgrade
this for linking in new programs to old ones.

Fixed intrinsic VPDP[SS,SU,UU]D[,S]_128/256/512's argument types to match with the ISA.
Fixes part of #97271.

The partial reduction intrinsics are no longer experimental, because
they've been used in production for a while and are unlikely to change.

Fixed intrinsic VPDPBUSD[,S]_128/256/512's argument types to match with the ISA.

Fixes part of #97271

In order to better see what's going on during ThinLTO linking, this PR
adds more profile tags when using `--time-trace` on a `lld-link.exe`
invocation.

After PR, linking `clang.exe`:

<img width="3839" height="2026" alt="Capture d’écran 2025-09-02 082021"
src="https://github.com/user-attachments/assets/bf0c85ba-2f85-4bbf-a5c1-800039b56910"
/>

Linking a custom (Unreal Engine game) binary gives a completly
different picture, probably because of using Unity files, and the sheer
amount of input files (here, providing over 60 GB of .OBJs/.LIBs).

<img width="1940" height="1008" alt="Capture d’écran 2025-09-02 102048"
src="https://github.com/user-attachments/assets/60b28630-7995-45ce-9e8c-13f3cb5312e0"
/>