[MachineLICM] Don't allow hoisting invariant loads across mem barrier. (#116987)

The improvements in 63917e19 / #70796 do not check for memory
barriers/unmodelled sideeffects, which means we may incorrectly hoist
loads across memory barriers.

Fix this by checking any machine instruction in the loop is a load-fold
barrier.

PR: https://github.com/llvm/llvm-project/pull/116987