Original commit: a61f9fe31750cee65c726fb51f1b14e31e177258
Multiple 2-stage buildbots were reporting failures. These issues have been
addressed separately.
Fixes: https://github.com/llvm/llvm-project/issues/87534.
A logic issue arose when inlining via `CloneAndPruneFunctionInto`,
which, besides cloning, performs instruction simplification as well.
By the time a new cloned instruction is being simplified, phi-nodes
are not remapped yet as the whole CFG needs to be processed first.
As `VMap` state at this stage is incomplete, `threadCmpOverPHI` and
variants could lead to unsound optimizations. This issue has been
addressed by performing basic constant folding while cloning, and
postponing instruction simplification once phi-nodes are revisited.
Fixes: https://github.com/llvm/llvm-project/issues/87534.
