[RegScavenger] Change scavengeRegister to pick registers in allocation order

This matches what scavengeRegisterBackwards does.

This is in preparation for converting most uses of scavengeRegister to
scavengeRegisterBackwards, to reduce test case churn when that lands and
to help with bisection if anything goes wrong.

Differential Revision: https://reviews.llvm.org/D150792