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)

src/intel/compiler/brw_compiler.c

@@ -41,6 +41,7 @@
    .lower_usub_borrow = true,                                                 \
    .lower_fdiv = true,                                                        \
    .lower_flrp64 = true,                                                      \
+   .lower_isign = true,                                                       \
    .lower_ldexp = true,                                                       \
    .lower_cs_local_id_from_index = true,                                      \
    .lower_device_index_to_zero = true,                                        \