[RegisterScavenging] Fix assert in scavengeRegisterBackwards

According to the documentation, if a spill is required to make a
register available and AllowSpill is false, then NoRegister should be
returned, however, this scenario was actually triggering an assertion
failure.

This patch moves the assertion after the handling of AllowSpill.

Authored by: Lewis Revill

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D92104