Simplify EXTRACT_SUBREG emission.

EXTRACT_SUBREG is emitted as %dst = COPY %src:sub, so there is no need to
constrain the %dst register class.  RegisterCoalescer will apply the
necessary constraints if it decides to eliminate the COPY.

The %src register class does need to be constrained to something with
the right sub-registers, though.  This is currently done manually with
COPY_TO_REGCLASS nodes.  They can possibly be removed after this patch.

llvm-svn: 141207