nir/opcodes: Fix constant-folding of ufind_msb

We didn't fold correctly in the case of 0x1 because we never let the
loop counter hit 0.  Switching it to bit >= 0 solves this problem.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Cc: mesa-stable@lists.freedesktop.org

src/compiler/nir/nir_opcodes.py

@@ -308,7 +308,7 @@ for (unsigned bit = 0; bit < 32; bit++) {
 
 unop_convert("ufind_msb", tint32, tuint32, """
 dst = -1;
-for (int bit = 31; bit > 0; bit--) {
+for (int bit = 31; bit >= 0; bit--) {
    if ((src0 >> bit) & 1) {
       dst = bit;
       break;