[VPlan] Don't rely on region check in isUniformAfterVectorization. (#137883)
Generalize isUniformAfterVectorization check to not rely on the region,
but purely work on checking operands and opcodes.
This will be needed when disolving the vector region
(https://github.com/llvm/llvm-project/pull/117506) and improves codegen
slightly in some cases.
PR: https://github.com/llvm/llvm-project/pull/137883