[llvm] GlobalISel: Add m_GIntrinsic matcher (PR #217439)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Shall we add unit test for new gisel matcher here llvm/unittests/CodeGen/GlobalISel/PatternMatchTest.cpp ?

In general I'd say no, but I thought about it for this case in particular because the argument discard behavior is kind of weird but apparently matches the IR version 

https://github.com/llvm/llvm-project/pull/217439