Register coalescing can change the target of a RegPair hint to a
physreg, we should not crash on this. This also slightly improved the
way ARMBaseRegisterInfo::updateRegAllocHint() works.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233987 91177308-0d34-0410-b5e6-96231b3b80d8