nir: fix constant expression of ibitfield_extract
This fixes dEQP-VK.graphicsfuzz.cov-condition-bitfield-extract-integer. For example, nir_ibitfield_extract(3, 1, 2) should return 1. Cc: 21.3 mesa-stable Signed-off-by:Samuel Pitoiset <samuel.pitoiset@gmail.com> Reviewed-by:
Rhys Perry <pendingchaos02@gmail.com> Part-of: <mesa/mesa!13791> (cherry picked from commit 011ea325)