[RISCV] Custom lower vector ISD::CTLZ to the minimum of ISD::CTLZ_ZERO_UNDEF and element size.

D111904 made RISC-V customized lower ISD::CTLZ_ZERO_UNDEF by converting to float
and using the float result. The expected value of CTLZ with zero input is the
element size of input type. Since the result of above method with zero input
must be greater than the element size, for ISD::CTLZ, we could use the minimum
of element size and the result of CTLZ_ZERO_UNDER with same input.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D141585