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

[John Stuart via llvm-commits]

john-stuart2 wrote:

The combiner holds higher standards when it comes to legality. You must check for legality before building. The constant folding uses an optimistic approach. It will just work. For asymmetric opcodes like G_SITOFP,  G_CTLZ, and G_ICMP I am not convinced that this is the right approach.

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