[TableGen] Use 'size_t' instead of 'unsigned' to better match the argument...

[TableGen] Use 'size_t' instead of 'unsigned' to better match the argument types of addAsmOperand. Simplify some code by using StringRef::find instead of std::find. These were previously done in r247527 and r247528, but another commit seems to have erased them. NFC

llvm-svn: 256624