[MergeICmps] MergeICmps is a new optimization pass that turns chains of integer

comparisons into memcmp.

Thanks to recent improvements in the LLVM codegen, the memcmp is typically
inlined as a chain of efficient hardware comparisons.
This typically benefits C++ member or nonmember operator==().

For now this is disabled by default until:
 - https://bugs.llvm.org/show_bug.cgi?id=33329 is complete
 - Benchmarks show that this is always useful.

Differential Revision:
https://reviews.llvm.org/D33987

llvm-svn: 312315