[llvm] [GlobalISel] Fold G_ICMP if possible (PR #86357)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> This sounds reasonable to me but I don't think we should enforce that everywhere, because we use defined regs in the instruction selector pass. In AArch64 we often use MachineIRBuilder's `buildInstr()`, the non-CSE version that is, to easily generate target specific instructions.

I don't think the selector should be using MachineIRBuilder like this, and we shouldn't design API around this usage. It's not really saving you much over using a raw BuildMI call 

https://github.com/llvm/llvm-project/pull/86357