Teach tblgen to accept register source operands in patterns, e.g.

def SHL8rCL  : I<0xD2, MRM4r, (ops R8 :$dst, R8 :$src),
                 "shl{b} {%cl, $dst|$dst, %CL}",
                 [(set R8:$dst, (shl R8:$src, CL))]>, Imp<[CL],[]>;

This generates a CopyToReg operand and added its 2nd result to the shl as
a flag operand.

llvm-svn: 24557