[RISCV] Custom type legalize i32 ISD::ABS on RV64 without Zbb.

Default type legalization will create sext_inreg+abs, but we may
not be able to remove the sext_inreg.

Instead this patch expands abs during type legalization to
Y = sraiw X, 31; subw(xor X, Y), Y) which doesn't require the input
to be sign extended.

This gives a big improvement for some neg-abs tests where the
abs is used more than the the neg. Previously the abs was expanded
a different way before and after type legalization. Now they are
expanded in a similar way enabling more CSE.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D120636