Cache result of an expensive DCHECK

LiveInterval::AddBackEdgeUses tests whether linear order is well
formed on debug builds. This is expensive and can significantly hinder
compilation times when many back edge uses are added.

This patch moves the IsLinearOrderWellFormed test at the end of
linear order generation.

Bug: 31163119
Change-Id: Ic4fe66bee2055f4b2cb065d9451ad5f21ba00676
2 files changed