[InstCombine] Canonicalize SPF to abs intrinsic

This patch enables canonicalization of SPF_ABS and SPF_ABS
to the abs intrinsic.

This is a recommit, the original try was
05d4c4eb,
but it was reverted due to an apparent miscompile,
which since then has just been fixed by the previous commit.

Differential Revision: https://reviews.llvm.org/D87188