[MachineRegisterInfo] Avoid having dbg.values affect code generation

Summary:
Use use_nodbg_empty() rather than use_empty() in
MachineRegisterInfo::EmitLiveInCopies() when determining if a livein
register has any uses or not. Otherwise a single dbg.value can make us
generate different code, meaning -g would affect code generation.

Found when compiling code for my out-of-tree target. Unfortunately I
haven't been able to reproduce the problem on X86 or any of the other
in-tree targets that I tried, so no test case.

Reviewers: MatzeB

Reviewed By: MatzeB

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39044

llvm-svn: 318382