[LiveDebugValues] Visit open var locs just once in transferRegisterDef, NFC

For a file in WebKit, this brings the time spent in LiveDebugValues down
from 16 minutes to 2 minutes. The reduction comes from iterating the set
of open variable locations just once in transferRegisterDef. Post-patch,
the most expensive item inside of transferRegisterDef is a call to
VarLoc::isDescribedByReg, which we have to do.

Testing: I built LNT using the Os-g cmake cache with & without this
patch, then diffed the object files to verify there was no binary diff.

rdar://59446577

Differential Revision: https://reviews.llvm.org/D74633