[ADT] Switch a bunch of places in LLVM that were doing single-character

[ADT] Switch a bunch of places in LLVM that were doing single-character
splits to actually use the single character split routine which does
less work, and in a debug build is *substantially* faster.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247245 91177308-0d34-0410-b5e6-96231b3b80d8