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: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13791>

src/compiler/nir/nir_opcodes.py

@@ -1056,7 +1056,7 @@ if (bits == 0) {
 } else if (offset < 0 || bits < 0 || offset + bits > 32) {
    dst = 0;
 } else {
-   dst = (base << (32 - offset - bits)) >> offset; /* use sign-extending shift */
+   dst = (base << (32 - offset - bits)) >> (32 - bits); /* use sign-extending shift */
 }
 """)