[X86] Don't emit COPY_TO_REG to ABCD registers before EXTRACT_SUBREG of sub_8bit

This is similar to D37843, but for sub_8bit. This fixes all of the patterns except for the 2 that emit only an EXTRACT_SUBREG. That causes a verifier error with global isel because global isel doesn't know to issue the ABCD when doing this extract on 32-bits targets.

Differential Revision: https://reviews.llvm.org/D37890

llvm-svn: 313558