[TargetLowering][RISCV][Sparc] Don't emit zero check in CTTZTableLookup for CTTZ_ZERO_UNDEF.

The code incorrectly checked for CTLZ_ZERO_UNDEF instead of
CTTZ_ZERO_UNDEF.

While I was there I flipped the condition into an early out.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D136010