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
55f9eccee9