[LiveDebugValues] Remove early-exit when testing regmasks, NFC

In transferRegisterDef, if the instruction has a regmask attached, we'll
check if any currently used register is clobbered by the regmask.

The early exit in this scan isn't necessary, costs a set lookup, and is
almost never taken [1]. Delete it.

[1]
http://lab.llvm.org:8080/coverage/coverage-reports/coverage/Users/buildslave/jenkins/workspace/coverage/llvm-project/llvm/lib/CodeGen/LiveDebugValues.cpp.html#L1136