We are no able to inline OpKill instructions into a continue construct.
See #2433.  However, we have to be able to inline to correctly do
legalization.  This commit creates a pass that will wrap OpKill
instructions into a function of its own.  That way we are able to inline
the rest of the code.

The follow up to this will be to not inline any function that contains
an OpKill.

Fixes #2726

Fixes #2793

* Don't special case matrix validation compared to other composites
  * just check the constituents are constants or undefs
  * later checking validates the column type
  * new test

`#include <source/util/string_utils.h>` works only when we specify
`include_directories(${CMAKE_CURRENT_SOURCE_DIR}/)` in
cmake. It is hard to set the source directory as a include path
in some build systems e.g., bazel. Using the relative path easily
solves this issue. This commit uses
`#include "source/util/string_utils.h"` instead of
`#include <source/util/string_utils.h>`.

My local python reports that CHANGES uses a non-ascii code
character and fail in running utils/update_build_version.py.

… Specification (#2791)

If we run out of ids when creating a new variable, sroa does not recognize
the error, and continues doing work.  This leads to segmentation faults.

Fixes https://crbug/969655

Fixes https://crbug/994203