[RegAllocFast] Handle new debug values for spills

These new debug values get inserted after the place where the spill
happens, which means they won't be reached by the reverse traversal of
basic block instructions. This would crash or fail assertions if they
contained any virtual registers to be replaced. We can manually handle
the new debug values right away to resolve this.

Fixes https://github.com/llvm/llvm-project/issues/59172

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D139590

(cherry picked from commit 87f57f45)