Allow -inline-threshold to override default threshold.

Before r257832, the threshold used by SimpleInliner was explicitly specified or generated from opt levels and passed to the base class Inliner's constructor. There, it was first overridden by explicitly specified -inline-threshold. The refactoring in r257832 did not preserve this behavior for all opt levels. This change brings back the original behavior.

Differential Revision: http://reviews.llvm.org/D20452

llvm-svn: 270153