[CodeGen] Compute DT/LI lazily in SafeStackLegacyPass. NFC.

We currently require SCEV, which requires DT/LI.  Those are expensive to
compute, but the pass only runs for functions that have the safestack
attribute.

Compute DT/LI to build SCEV lazily, only when the pass is actually going
to transform the function.

Differential Revision: https://reviews.llvm.org/D31302

llvm-svn: 302610