der: reduce llvm-ir in SliceReader::read_nested

[dishmaker]

Before:
   2241 (3.7%,  8.6%)    12 (0.9%,  2.5%)  <der::reader::slice::SliceReader as der::reader::Reader>::read_nested
After:
   1639 (2.8%, 14.7%)    12 (0.9%, 10.0%)  <der::reader::slice::SliceReader as der::reader::Reader>::read_nested

[baloo]

Curious: how do you get that diff output?

#1799 (comment)