[RISCV][WebAssembly][TargetLowering] Allow expandCTLZ/expandCTTZ to rely on...

[RISCV][WebAssembly][TargetLowering] Allow expandCTLZ/expandCTTZ to rely on CTPOP expansion for vectors.

Our fallback expansion for CTLZ/CTTZ relies on CTPOP. If CTPOP
isn't legal or custom for a vector type we would scalarize the
CTLZ/CTTZ. This is different than CTPOP itself which would use a
vector expansion.

This patch teaches expandCTLZ/CTTZ to rely on the vector CTPOP
expansion instead of scalarizing. To do this I had to add additional
checks to make sure the operations used by CTPOP expansions are all
supported. Some of the operations were already needed for the CTLZ/CTTZ
expansion.

This is a huge improvement to the RISCV which doesn't have a scalar
ctlz or cttz in the base ISA.

For WebAssembly, I've added Custom lowering to keep the scalarizing
behavior. I've also extended the scalarizing to CTPOP.

Differential Revision: https://reviews.llvm.org/D111919