Integrate LLVM at llvm/llvm-project@f6d0a512972a

Updates LLVM usage to match
[f6d0a512972a](https://github.com/llvm/llvm-project/commit/f6d0a512972a)

PiperOrigin-RevId: 855954028

Integrate LLVM at llvm/llvm-project@21a1e6e6a70d

Updates LLVM usage to match
[21a1e6e6a70d](https://github.com/llvm/llvm-project/commit/21a1e6e6a70d)

PiperOrigin-RevId: 855485902

This change made single-argument constructors explicit.

Making single-argument constructors explicit avoids unintentional implicit conversions.

PiperOrigin-RevId: 854188570

[codegen 1.5] Port & convert module signature generation to a pass

This pass runs on all blocks without signatures in a package and populates them. The latency calculation for pipelined blocks is simplified to use the schedule information, if present.

For signature preservation, we also update block cloning to clone the signature and initiation interval.

PiperOrigin-RevId: 853909739

[codegen 1.5] Improve handling of `invoke` nodes, working towards FFI…

[codegen 1.5] Add reset requirement for valid-output control

If lowering a function with a valid-output control signal, we should always have a reset signal; otherwise we can end up with (pipeline_depth - 1) cycles of garbage at the valid-output signal, potentially signaling "valid" when the output is very much not valid. This required adding reset signal support to our unit tests' RunFunctionalTest logic for this pass.

While we're at it, we also ensure that the source return value is always available in the last pipeline stage by inserting an identity node if necessary - which can be necessary when lowering a no-op function that directly returns one of its inputs. (This mostly only comes up in testing.)

PiperOrigin-RevId: 853045608

Enable previously-failing crasher.

Fixes google/xls#3269.

PiperOrigin-RevId: 852441042

Integrate LLVM at llvm/llvm-project@11d9694b757b

Updates LLVM usage to match
[11d9694b757b](https://github.com/llvm/llvm-project/commit/11d9694b757b)

PiperOrigin-RevId: 852027392

[codegen 1.5] Make channel lowering respect placeholders

Our node utility function ReplaceWithAnd previously unconditionally dropped all-ones literals - which caused some substantial errors in context of Codegen 1.5's use of literals as placeholders. We instead explicitly block this behavior, while still leaving the utility functions fully flexible for convenience.

While we're at it, we upgrade the utility functions to be able to combine arbitrary literals into a single literal when requested.

PiperOrigin-RevId: 851840930

[codegen 1.5] Fix a pipeline-register insertion bug for I/O & state o…