Skip debuginfo intrinsic in markLiveBlocks.

Summary:
The optimizer is 10%+ slower with vs without debuginfo. I started checking where
the difference is coming from.

I compiled sqlite3.c with and without debug info from CTMark and compare the time difference.

I use Xcode Instrument to find where time is spent. This brings about 20ms, out of ~20s.

Reviewers: davide, hfinkel

Reviewed By: hfinkel

Subscribers: hfinkel, aprantl, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D49337

llvm-svn: 337416