in the case where an instruction only has one implementation

of a mneumonic, report operand errors with better location
info.  For example, we now report:

t.s:6:14: error: invalid operand for instruction
        cwtl $1
             ^

but we fail for common cases like:

t.s:11:4: error: invalid operand for instruction
   addl $1, $1
   ^

because we don't know if this is supposed to be the reg/imm or imm/reg
form.

llvm-svn: 113178