[InstCombine] Canonicalize SPF_ABS to abs intrinc

Enable canonicalization of SPF_ABS and SPF_NABS to the abs intrinsic.

To be conservative, the one-use check on the comparison is retained,
this may be relaxed if all goes well.

It's pretty likely that this will uncover places that missing
handling for the abs() intrinsic. Please report any seen performance
regressions.

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