[RISCV] Lower VP_CTLZ_ZERO_UNDEF/VP_CTTZ_ZERO_UNDEF/VP_CTLZ by converting to...

[RISCV] Lower VP_CTLZ_ZERO_UNDEF/VP_CTTZ_ZERO_UNDEF/VP_CTLZ by converting to FP and extracting the exponent.

D111904, D141585 made RISC-V customized lower vector ISD::CTLZ_ZERO_UNDEF/CTTZ_ZERO_UNDEF/CTLZ
by converting to float and using the float result.

Perhaps VP_CTLZ_ZERO_UNDEF/VP_CTTZ_ZERO_UNDEF/VP_CTLZ could use the similar feature.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D155150