[UnreachableBlockElim] Check return value of constrainRegClass().
Summary:
MachineRegisterInfo::constrainRegClass() can fail if two register classes
don't have a common subclass or if the register class doesn't contain
enough registers. Check the return value before trying to remove Phi nodes,
and if we can't constrain, we output a COPY instead of simply replacing
registers.
Reviewers: kparzysz, david2050, wmi
Reviewed By: kparzysz
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D32999
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302622
91177308-0d34-0410-b5e6-
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