[globalisel][combine] Fix a rare crash when encountering an instruction whose op0 isn't a reg

The simplest instance of this is an intrinsic with no results which will have the
intrinsic ID as operand 0.

Also fix some benign incorrectness when op0 is a reg but isn't a def that was
guarded against by checking for the extension opcodes.

llvm-svn: 343821