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

Summary:
This function was performing two hash lookups when a new struct 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 struct type was not there before.

Similar to r345151.

Reviewers: bkramer

Reviewed By: bkramer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D53689

llvm-svn: 345264