[RISCV] With Zbb, fold (sext_inreg (abs X)) -> (max X, (negw X))

With Zbb, abs is expanded to (max X, neg) by default. If X has 33 or
more sign bits, we can expand it a little early using negw instead of
neg to save a sext_inreg. If X started as a 32 bit value, type
legalization would have inserted a sext before the abs so X having
33 sign bits should always be true.

Note: I've used ISD::FREEZE here since we increase the number of uses.
Our default expansion for ABS doesn't do that, but I think that's a bug.

We can't do this with custom type legalization because ISD::FREEZE
doesn't propagate sign bits so later DAG combine won't expand be
able to see optmize it.

Alives2 https://alive2.llvm.org/ce/z/Gx3RNe

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D120597

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