[tablegen] Improve performance of -gen-register-info by replacing...

[tablegen] Improve performance of -gen-register-info by replacing barely-necessary std::map with a sorted vector

Summary:
This particular map is hardly ever queried and has a phased usage pattern (insert,
iterate, query, insert, iterate) so it's a good candidate for a sorted vector and
std::lower_bound.

This significantly reduces the run time of runTargetDesc() in some circumstances.
One llvm-tblgen invocation in my build improves the time spent in runTargetDesc()
from 9.86s down to 0.80s (~92%) without changing the output. The same invocation
also has 2GB less allocation churn.

Reviewers: bogner, rtereshin, aditya_nandakumar, volkan

Reviewed By: rtereshin

Subscribers: mgrang, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D50272

llvm-svn: 339208