Fix instruction scheduling live register tracking

Summary:
This patch fixes PR23405 (https://llvm.org/bugs/show_bug.cgi?id=23405).

During a node unscheduling an entry in LiveRegGens can be replaced with a new value. That corrupts the live reg tracking and LiveReg* structure is not cleared as should be during unscheduling. Problematic condition that enforces Gen replacement is `I->getSUnit()->getHeight() < LiveRegGens[I->getReg()]->getHeight()`. This condition should be checked only if LiveRegGen was set in current node unscheduling.

Test Plan: Regression test included.

Reviewers: hfinkel, atrick

Reviewed By: atrick

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9993

llvm-svn: 240538