[llvm] [RFC][GlobalISel] Use Builders in MatchTable (PR #65955)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

> I'm not really a fan of observers. Why does the match table need these? The execution should know precisely what it's doing without tracking other dynamic context?

We still use C++ most of the time, and it's impossible for the MatchTable to know what's going on in there unless it uses an observer, so I thought it'd be a nice change to use an observer for tracking all changes.
Likewise, when we use builder functions, like in #66077, we have no way of knowing how many insts were actually built without using a builder.

We can do without the builder, but then flag propagation will be a bit broken.

https://github.com/llvm/llvm-project/pull/65955