In 64-bit mode, sub_8bit_hi sub-registers can only be used by NOREX
instructions. The COPY created from the EXTRACT_SUBREG DAG node cannot
target all GR8 registers, only those in GR8_NOREX.

TO enforce this, we ensure that all instructions using the
EXTRACT_SUBREG are GR8_NOREX constrained.

This fixes PR11088.

llvm-svn: 141499