Revert "[AssumptionCache] Avoid dangling llvm.assume calls in the cache"

This reverts commit b7d870ea and the
subsequent fix "[Polly] Fix build after AssumptionCache change (D96168)"
(commit e6810cab).

It caused indeterminism in the output, such that e.g. the
polly-x86_64-linux buildbot failed accasionally.