intel: Use the NIR lowering for isign.

intel: Use the NIR lowering for isign.

Drops one instruction from fs-sign-int.shader_test.  No change in
shader-db due to it having 0 instances of sign(genIType).  This may hurt
isign64 if algebraic runs before int64 lowering, but I wasn't sure how to
mark the algebraic opt as "every bit size but 64".

v2: Update commit message about shader-db.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> (v1)