[AMDGPU] SILowerControlFlow: fix preservation of LiveIntervals

In emitElse live interval for SI_ELSE source must be recalculated
as SI_ELSE is removed, and new user is placed at block start.
In emitIfBreak live interval for new created AndReg must be
computed.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D158141