ac/nir: do not sign-extend the result of texop_samples_identical

The return value should be a NIR 1-bit bool.

This fixes a regression with piglit
ext_shader_samples_identical-simple-fs 2 on RadeonSI.

Fixes: e690a1b78bf ("ac/llvm: don't lower bool to int32, switch to native i1 bool")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7236>