https://github.com/llvm/llvm-project/pull/83381 introduced a call
to PHINode::isComplete() in Mem2Reg, which is O(n^2) in the number
of predecessors, resulting in pathological compile-time regressions
for cases with many predecessors.
Remove the isComplete() check and instead cache the attribute
lookup, to only perform it once per function. Actually setting
the FMF flag is cheap.
9f8f6ce53c