[LoongArch] Fixing the incorrect return value of LoongArchTTIImpl::getRegisterBitWidth (#79441)

When we do not enable vector features, we should return the default
value (`TargetTransformInfoImplBase::getRegisterBitWidth`) instead of
zero.

This should fix the LoongArch [buildbot
breakage](https://lab.llvm.org/staging/#/builders/5/builds/486) from
#78943.