Do not make -pass-remarks additive.

Summary:
When I initially introduced -pass-remarks, I thought it would be a
neat idea to make it additive. So, if one used it as:

$ llc -pass-remarks=inliner --pass-remarks=loop.*

the compiler would build the regular expression '(inliner)|(loop.*)'.

The more I think about it, the more I regret it. This is not how
other flags work. The standard semantics are right-to-left overrides.

This is how clang interprets -Rpass. And I think the two should be
compatible in this respect.

Reviewers: qcolombet

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3614