Fix for InlineSpiller accessing not updated dom tree base information.

Summary:
The InlineSpiller was accessing the DominatorTreeBase directly
through the public data member DT in the MachineDominatorTree.
This is not a good idea as the "cached" information in
SplitCriticalEdges is not applied before the access.
The DominatorTreeBase must be accessed through the member
function getBase() in MachineDominatorTree.

The fault was introduced in r266162.

I think the public data member DT in the MachineDominatorTree
should have been made private in the original code (r215576)
that introduced the concept of lazily updating the
MachineDominatorTree information from
MachineBasicBlock::SplitCriticalEdge().

Patch by Karl-Johan Karlsson <karl-johan.karlsson@ericsson.com>

Reviewers: wmi, qcolombet

Subscribers: llvm-commits, bjope, uabelho

Differential Revision: https://reviews.llvm.org/D27983

llvm-svn: 290950