Revert "intel/fs: Move the scalar-region conversion to the generator."

This reverts commit c0504569ea.  Now that
we're doing interpolation lowering in NIR, we can continue to stride the
FS input registers directly in the brw_fs_nir code like we did before.
This fixes SIMD32 fragment shaders which broke because lower_simd_width
depended on the 0 stride to split PLN instructions correctly.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>

src/intel/compiler/brw_fs.cpp

@@ -1192,7 +1192,7 @@ fs_visitor::emit_fragcoord_interpolation(fs_reg wpos)
    } else {
       bld.emit(FS_OPCODE_LINTERP, wpos,
                this->delta_xy[BRW_BARYCENTRIC_PERSPECTIVE_PIXEL],
-               interp_reg(VARYING_SLOT_POS, 2));
+               component(interp_reg(VARYING_SLOT_POS, 2), 0));
    }
    wpos = offset(wpos, bld, 1);