[ScheduleDAGInstrs] fix behavior of getUnderlyingObjectsForCodeGen when no identifiab...

[ScheduleDAGInstrs] fix behavior of getUnderlyingObjectsForCodeGen when no identifiable object found

This patch fixes the bug introduced in https://reviews.llvm.org/D35907; the bug is reported by http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20171002/491452.html.

Before D35907, when GetUnderlyingObjects fails to find an identifiable object, allMMOsOkay lambda in getUnderlyingObjectsForInstr returns false and Objects vector is cleared. This behavior is unintentionally changed by D35907.

This patch makes the behavior for such case same as the previous behavior.
Since D35907 introduced a wrapper function getUnderlyingObjectsForCodeGen around GetUnderlyingObjects, getUnderlyingObjectsForCodeGen is modified to return a boolean value to ask the caller to clear the Objects vector.

Differential Revision: https://reviews.llvm.org/D38735

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315565 91177308-0d34-0410-b5e6-96231b3b80d8