Do not specialize IRBuilder to strip names in SROA

Summary:
Following r263086, we are replacing this by a runtime check.
More cleanup will follow on the IRBuilder itself, but I submitted
this patch separately as SROA has a fancy "prefixInserter" class
that needs extra-love.

Reviewers: chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18022

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 263256