panfrost: Fix size assertion in bi_alu_src_index

Shifting by the bitsize was not only wrong, the shift is undefined
behavior when bitsize is 32, causing the assertion to fire on AArch32.

Fixes: 95d62ee7cf ("pan/bi: Add bi_alu_src_index helper")
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8460>

src/panfrost/bifrost/bifrost_compile.c

@@ -740,7 +740,7 @@ bi_alu_src_index(nir_alu_src src, unsigned comps)
         assert(idx.swizzle == BI_SWIZZLE_H01);
 
         /* Bigger vectors should have been lowered */
-        assert(comps <= (1 << bitsize));
+        assert(comps <= (1 << subword_shift));
 
         if (bitsize == 16) {
                 unsigned c0 = src.swizzle[0] & 1;