Previously PatLeafs could not be imported, causing the following
warnings to be emitted when running tblgen with
`-warn-on-skipped-patterns:`
```
/work/clean/llvm/lib/Target/AArch64/AArch64InstrInfo.td:2631:1: warning: Skipped pattern: Src pattern child has unsupported predicate
def : Pat<(i64 (mul top32Zero:$Rn, top32Zero:$Rm)),
^
```
These changes allow the patterns to now be imported successfully.

Rename `ListInit::getValues()` to `getElements()` to better match with
other `ListInit` members like `getElement`. Keep `getValues()` for
existing downstream code but mark it deprecated.

/llvm-project/llvm/utils/TableGen/GlobalISelCombinerEmitter.cpp:2612:14:
error: variable 'ExpectedID' set but not used [-Werror,-Wunused-but-set-variable]
    unsigned ExpectedID = 0;
             ^
1 error generated.

The increment must be after the assert, not before, otherwise the assert fails everytime.

ExpectedID should be optimized out anyway if built without assertions
because nothing reads its value.

Fixes #90327

Adds a `combine` action (DAG operator) which allows for easy definition of
combine rule that only match one or more instructions, and defer all remaining
match/apply logic to C++ code.

This avoids the need for split match/apply function in such cases. One function
can do the trick as long as it returns `true` if it changed any code.

This is implemented as syntactic sugar over match/apply. The combine rule is
just a match pattern BUT every C++ pattern inside is treated as an "apply" function.
This makes it fit seamlessly with the current backend.

Fixes #92410

Currently iterators over EquivalenceClasses will iterate over std::set,
which guarantees the order specified by the comperator. Unfortunately in
many cases, EquivalenceClasses are used with pointers, so iterating over
std::set of pointers will not be deterministic across runs.

There are multiple places that explicitly try to sort the equivalence
classes before using them to try to get a deterministic order
(LowerTypeTests, SplitModule), but there are others that do not at the
moment and this can result at least in non-determinstic value naming in
Float2Int.

This patch updates EquivalenceClasses to keep track of all members via a
extra SmallVector and removes code from LowerTypeTests and SplitModule
to sort the classes before processing.

Overall it looks like compile-time slightly decreases in most cases, but
close to noise:

https://llvm-compile-time-tracker.com/compare.php?from=7d441d9892295a6eb8aaf481e1715f039f6f224f&to=b0c2ac67a88d3ef86987e2f82115ea0170675a17&stat=instructions

PR: https://github.com/llvm/llvm-project/pull/134075