Fix for PR34888.

Fix for PR34888.

The issue is that we assume operand zero of the input to the add instruction
is a register. In this case, the input comes from inline assembly and
operand zero is not a register thereby causing a crash.
The code will bail anyway if the input instruction doesn't have the right
opcode. So do that check first and let short-circuiting prevent the crash.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315285 91177308-0d34-0410-b5e6-96231b3b80d8