Remove double-def checking from MachineVerifier, so a register does not have to

Remove double-def checking from MachineVerifier, so a register does not have to
be killed before being redefined.

These checks are usually disabled, and usually fail when enabled. We de facto
allow live registers to be redefined without a kill, the corresponding
assertions in RegScavenger were removed long ago.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110362 91177308-0d34-0410-b5e6-96231b3b80d8