[DAG] Add `SDPatternMatch::m_Load` (#145481)
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.
