loop-unswitch in the main pipelines for the new PM.

All of these now work, and Clang built using this pipeline can build the
test suite and SPEC without hitting any asserts of ASan failures.

There are still some bugs hiding though -- 7 tests regress with the new
PM. I'm going to be investigating these, but it seems worthwhile to at
least get the pipelines in place so that others can play with them, and
they aren't completely broken.

Differential Revision: https://reviews.llvm.org/D29113

llvm-svn: 293225