[llvm] [GlobalISel] An optimizing MIR builder (PR #132282)

[John Stuart via llvm-commits]

john-stuart2 wrote:

To give a bit more context, my dream is to remove constant folding from the CSEMIRBuilder and have a separate builder for constant folding. Please inspect the constructor of the new builder. The proposed builder uses undef, poison, and constant folding for optimizations. It delegates on failure to optimize to the CSEMIRBuilder. The main advertisement of the new builder is that always optimizes into legal gMIR. 

https://github.com/llvm/llvm-project/pull/132282