Tweak the ThinLTO pass pipeline

Summary:
The original ThinLTO pipeline was derived from some
work I did tuning FullLTO on the test suite and SPEC. This
patch reduces the amount of work done in the "linker phase" of
the build, and extend the function simplifications passes
performed during the "compile phase". This helps the build time
by reducing the IR as much as possible during the compile phase
and limiting the work to be performed during the "link phase",
while keeping the performance "on par" with the existing pipeline.

Reviewers: tejohnson

Subscribers: llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D19773

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 268769