[RISCV] Change the type of argument to clz and ctz from ZiZi/WiWi to iUZi/iUWi

Input argument of clz and ctz should be unsigned type and return value
should be integer like `builtin_clz` and `builtin_ctz` defined in clang/include/clang/Basic/Builtins.def.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D153235