[LV] Revert back to use Loop::isLoopInvariant in isPredicatedInst. (#150828)

This partially reverts https://github.com/llvm/llvm-project/pull/140744,
restoring the original TheLoop->isLoopInvariant check instead the more
powerful Legal->isInvariant, which uses SCEV.

This causes a mis-compile, because SCEV can prove that the stored value
is loop-invariant, which in turn converts the store to a uniform store.
But in VPlan, we aren't yet able to determine that the stored value is
loop-invariant, so we extract the last lane, which is incorrect, because
it does not account for the mask of the store.

Restoring the original code is a safe fix and avoids this subtle
divergence.

Fixes https://github.com/llvm/llvm-project/issues/149347.

PR: https://github.com/llvm/llvm-project/pull/150828

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