Avoid calls to dumpPassInfo and RegionBase<Tr>::getNameStr() in RGPassManager if

-debug-pass is not specified, as the string is only used when dumping pass
information.  There is a big cost of determining the name in
ReginBase<Tr>:getNameStr() if the region's entry or exit block doesn't have a
name.  This is the case for the Release build, as names are not preserved by the
front-end.

RegionPass is mainly used by Polly, resulting in long compile time for one file
of a customer application with the Release build (1m24s) vs Release+Asserts
build (10s) when Polly is used.  With this change, the compile time with the
Release build went down to 8s.

Patch by Sanjin Sijaric <ssijaric@codeaurora.org>!
Phabricator: http://reviews.llvm.org/D8076

llvm-svn: 231485