[MachineSink] Don't reject sinking because of dead def in isProfitableToSinkTo().

An instruction should be sunk (if otherwise legal and profitable) regardless
of if it has a dead def of a physreg or not. Physreg defs are checked in other
places and sinking is only done with dead defs of regs that are not live into
the target MBB.

Differential Revision: https://reviews.llvm.org/D150447

Reviewed By: sebastian-ne, arsenm