[MachineSink] Fix for breaking phi edges with instructions with multiple defs
BreakPHIEdge would be set based on whether the instruction needs to
insert a new critical edge to allow sinking into a block where the uses
are PHI nodes. But for instructions with multiple defs it would be reset
on the second def, allowing the instruciton to sink where it should not.
Differential Revision: https://reviews.llvm.org/D78087
(cherry picked from commit 44c4ba34d001dcf538d7396007b5611d6f697f86)