[llvm] [SimplifyCFG] When only one case value is missing, replace default with that case (PR #76669)

[David Blaikie via llvm-commits]

dwblaikie wrote:

> actually, this seems to be uncovering some bad behavior during codegen, still investigating

To expound a bit on this - the test case we're looking at goes from 300MB to 18GB peak RAM usage.

81% of that peak memory usage is coming from SSAUpdaterImpl. (TailDuplicateBase, TailDuplicator, MachineSSAUpdater)
(oh, and another 12% from the same place, basically - just BumpPtrAllocatorImpl for the operands rather than `new` for the MachineInstrs - and another 3% of more BumpPtrAllocatorImpl in the same place... so 96%)

https://github.com/llvm/llvm-project/pull/76669