Resolves https://github.com/llvm/llvm-project/issues/144477

---------

Co-authored-by: Simon Pilgrim <llvm-dev@redking.me.uk>

Add SDPatternMatch matcher and unit test coverage for `ISD::LOAD`
opcode.
This only matches the loaded value i.e. ResNo 0 and not the output
chain.
e.g.
```
m_Load(m_Value(), m_Value(), m_Value())
```

The first value is the input chain, the second is the base pointer, and
the last value is the offset.

`m_Result<N>` matches a SDValue that is the N-th result of the defining
SDNode. This is useful for creating a more fine-grained matching on
SDNode with multiple results.

-----
Inspired by #145481

Add SDPatternMatch matcher and unit test coverage for ISD::POISON opcode

e.g.
```
m_InsertElt(m_Poison(), m_Value(), m_Zero())
```

This patch transforms:

  X && *X == Y

to:

  X == Y

where X is of std::optional<T>, and Y is of T or similar.

fixes https://github.com/llvm/llvm-project/issues/118847

implements matchers for reassociatable opcodes as well as helpers for
commonly used reassociatable binary matchers.

---------

Co-authored-by: Min-Yih Hsu <min@myhsu.dev>

This avoids doing a Triple -> std::string -> Triple round trip in lots
of places, now that the Module stores a Triple.

Simplifies a future patch