[Instcombine] Disable memcpy of alloca bypass for instruction sources

This transformation is fundamentally broken when it comes to dominance,
it just happened to work when the source of the memcpy can be moved into
the place of the alloca. The bug shows up a lot more often since
077bff39 allows the source to be a
switch.

It would be possible to check dominance of the source and all its
operands, but that seems very heavy for instcombine.