in the case where an instruction only has one implementation

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@113178 91177308-0d34-0410-b5e6-96231b3b80d8