intel/reg,fs: Handle immediates properly in subscript()

Just returning the original type isn't what we want in basically any
case.  Mask and shift the immediate as needed.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7329>

src/intel/compiler/brw_ir_fs.h

@@ -303,8 +303,12 @@ subscript(fs_reg reg, brw_reg_type type, unsigned i)
       reg.vstride += (reg.vstride ? delta : 0);
 
    } else if (reg.file == IMM) {
-      assert(reg.type == type);
-
+      unsigned bit_size = type_sz(type) * 8;
+      reg.u64 >>= i * bit_size;
+      reg.u64 &= BITFIELD64_MASK(bit_size);
+      if (bit_size <= 16)
+         reg.u64 |= reg.u64 << 16;
+      return retype(reg, type);
    } else {
       reg.stride *= type_sz(reg.type) / type_sz(type);
    }