Implement some micro-optimizations for Identifier. NFC

Summary:
Identifier doesn't maintain a length, so every time strref() is called,
it does a strlen.  In the case of comparisons, this isn't necessary:
there is no need to scan a string to get its length, then rescan it to
do the comparison.  Just done one comparison.

This also moves some assertions in Identifier::get as another
microoptimization for 'assertions enabled' modes.

Reviewers: rriddle!

Subscribers: mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, grosul1, frgossen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77958