Now that we know if we had a total fail on the instruction mnemonic,

give a more detailed error.  Before:

t.s:11:4: error: unrecognized instruction
   addl $1, $1
   ^
t.s:12:4: error: unrecognized instruction
   f2efqefa $1
   ^

After:

t.s:11:4: error: invalid operand for instruction
   addl $1, $1
   ^
t.s:12:4: error: invalid instruction mnemonic 'f2efqefa'
   f2efqefa $1
   ^

This fixes rdar://8017912 - llvm-mc says "unrecognized instruction" when it means "invalid operands"

llvm-svn: 113176