[SelectionDAG][RISCV] Use isSExtCheaperThanZExt to control whether sext or zext is used for constant folding any_extend.

RISCV would prefer a sign extended constant since that works better
with our constant materialization. We have an existing TLI hook we
use to control sign extension of setcc operands in type legalization.
That hook happens to do the right check we need here, but might be
straying from its original purpose. With only RISCV defining this
hook in tree, I wasn't sure if it was worth adding another hook
with identical behavior.

This is an alternative to D105785 where I tried to handle this in
the RISCV backend by not creating ANY_EXTENDs in some places.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D105918

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