Update PPCRegisterInfo's use of virtual registers to be SSA

PPC's use of PEI's virtual-register-based scavenging functionality had
redefined the virtual registers (it was non-SSA). Now that PEI supports
dealing with instructions with multiple virtual registers, this can be
cleanup up to use multiple virtual registers and keep SSA form.

No functionality change intended.
Use multiple virtual registers in PPC CR spilling

Now that the register scavenger can support multiple spill slots, and PEI can
use virtual-register-based scavenging for multiple simultaneous registers, we
can use a virtual register for the transfer register in the CR spilling code.

This should eliminate the last place (outside of the prologue/epilogue) where
we depend on the unconditional availability of the r0 register. We will soon be
able to allocate it (in a somewhat restricted sense) as a GPR.
clang/test/CodeGenCXX/debug-info-namespace.cpp: Disable it on non-bash lit for now.

With dosish filename, it misgenerates an improper extra entry.

!1 = metadata !{metadata !"E:\5Cllvm\5Cllvm-project\5Cclang\5Ctest\5CCodeGenCXX/debug-info-namespace.cpp", metadata !"E:\5Cllvm\5Cbuild\5Cninja-win32-vs11\5Ctools\5Cclang\5Ctest\5CCodeGenCXX"}

!8 = metadata !{metadata !"E:\5Cllvm\5Cllvm-project\5Cclang\5Ctest\5CCodeGenCXX\5Cdebug-info-namespace.cpp", metadata !"E:\5Cllvm\5Cbuild\5Cninja-win32-vs11\5Ctools\5Cclang\5Ctest\5CCodeGenCXX"}

!8 is unexpected.
[analyzer] micro optimization as per Jordan’s feedback on r177905.