[ShrinkWrap] check for PPC's non-callee-saved LR

As pointed out in https://reviews.llvm.org/D115688#inline-1108193, we
don't want to sink the save point past an INLINEASM_BR, otherwise
prologepilog may incorrectly sink a prolog past the MBB containing an
INLINEASM_BR and into the wrong MBB.

ShrinkWrap is getting this wrong because LR is not in the list of callee
saved registers. Specifically, ShrinkWrap::useOrDefCSROrFI calls
RegisterClassInfo::getLastCalleeSavedAlias which reads
CalleeSavedAliases which was populated by
RegisterClassInfo::runOnMachineFunction by iterating the list of
MCPhysReg returned from MachineRegisterInfo::getCalleeSavedRegs.

Because PPC's LR is non-allocatable, it's NOT considered callee saved.
Add an interface to TargetRegisterInfo for such a case and use it in
Shrinkwrap to ensure we don't sink a prolog past an INLINEASM or
INLINEASM_BR that clobbers LR.

Reviewed By: jyknight, efriedma, nemanjai, #powerpc

Differential Revision: https://reviews.llvm.org/D116424