LegalizeDAG: allow libcalls for max/min atomic operations
ARM processors without ldrex/strex need to be able to make libcalls for all
atomic operations, including the newer min/max versions.
The alternative would probably be expanding these operations in terms of
cmpxchg (as x86 does always), but in the configurations where this matters
code-size tends to be paramount so the libcall is more desirable.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193398
91177308-0d34-0410-b5e6-
96231b3b80d8