Merging r329300:

------------------------------------------------------------------------
r329300 | manojgupta | 2018-04-05 08:29:52 -0700 (Thu, 05 Apr 2018) | 16 lines

Disable -fmerge-all-constants as default.

Summary:
"-fmerge-all-constants" is a non-conforming optimization and should not
be the default. It is also causing miscompiles when building Linux
Kernel (https://lkml.org/lkml/2018/3/20/872).

Fixes PR18538.

Reviewers: rjmccall, rsmith, chandlerc

Reviewed By: rsmith, chandlerc

Subscribers: srhines, cfe-commits

Differential Revision: https://reviews.llvm.org/D45289
------------------------------------------------------------------------

llvm-svn: 333564