On SNB, renamer-based zeroing does not work for:
- 16 and 8-bit GPRs[1].
- MMX [2].
- ANDN variants [3]
[1] echo 'sub %ax, %ax' | /tmp/llvm-exegesis -mode=uops -snippets-file=-
[2] echo 'pxor %mm0, %mm0' | /tmp/llvm-exegesis -mode=uops -snippets-file=-
[3] echo 'andnps %xmm0, %xmm0' | /tmp/llvm-exegesis -mode=uops -snippets-file=-