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

[John Stuart via llvm-commits]

john-stuart2 wrote:

I removed G_ADD, G_SUB, and G_MUL from the constant folding list in CSEMIRBuilder. It caused a **lot** of regressions. People depend on the constant folding in the builder.

GConstant can only provide helpers for scalars. How do you constant fold vectors with out much burden for the user?

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