There were a lot of std::vectors in the rewriter and in the invoke
machinery (callFunc and friends), and every std::vector usage involves
a call to malloc (and free when is destroyed.)  we should be using
llvm::SmallVector wherever we can in performance sensitive code, since it
allows a configurable stack allocated buffer.  It reverts to malloc/free
if you blow the buffer's capacity, but as long as things are tuned well,
we can get a pretty significant speedup.

There is more work to be done, but this change gets us ~3% on geomean.