[llvm] [SDPatternMatch] Add Matcher m_Undef (PR #122521)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

I mentioned the use case on #122439 - the place I hit this was I wanted to match the pattern below and it annoyed me as using m_Opc(ISD::UNDEF) broke the col80 limit:
```
// subvector widening pattern
m_InsertSubVector(m_Undef(), m_Value(), m_Zero())
```

https://github.com/llvm/llvm-project/pull/122521