[LegalizeTypes][RISCV][PowerPC] Expand CTLZ/CTTZ/CTPOP instead of promoting if...

[LegalizeTypes][RISCV][PowerPC] Expand CTLZ/CTTZ/CTPOP instead of promoting if they'll be expanded later.

Expanding these requires multiple constants. If we promote during type
legalization when they'll end up getting expanded in LegalizeDAG, we'll
use larger constants. These constants may be harder to materialize.
For example, 64-bit constants on 64-bit RISCV are very expensive.

This is similar to what has already been done to BSWAP and BITREVERSE.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D112268