[RegisterClassInfo] Return non-zero for RC without allocatable reg

In some case, the RC may have 0 allocatable reg.
eg: VRSAVERC in PowerPC, which has only 1 reg, but it is also reserved.

The curreent implementation will keep calling the computePSetLimit because
getRegPressureSetLimit assume computePSetLimit will return a non-zero value.

The fix simply early return the value from TableGen for such special case.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D92907