RegisterPressure: Factor out liveness dead-def detection logic; NFCI

RegisterPressure: Factor out liveness dead-def detection logic; NFCI

Detecting additional dead-defs without a dead flag that are only visible
through liveness information should be part of the register operand
collection not intertwined with the register pressure update logic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255192 91177308-0d34-0410-b5e6-96231b3b80d8