IR: Optimize FunctionType::get to perform one hash lookup instead of two, NFCI

Summary: This function was performing two hash lookups when a new function type was requested: first checking if it exists and second to insert it. This patch updates the function to perform a single hash lookup in this case by updating the value in the hash table in-place in case the function type was not there before.

Reviewers: bkramer

Reviewed By: bkramer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D53471

llvm-svn: 345151