Recommit "[LICM] Only create load in ph when promoting load or store doesn't exec."

This reverts the revert commit ad95255b.

The updated version also creates a load when the store may not execute.
In those cases, we still need to introduce a load in a function where
there may not have been one before, so this doesn't completely resolve
issue #51248.

Original message:

    When only a store is sunk, there is no need to create a load in the
    pre-header, as the result of the load will never get used.

    The dead load can can introduce UB, if the function is marked as
    writeonly.

    Reviewed By: nikic

    Differential Revision: https://reviews.llvm.org/D123473