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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256624 91177308-0d34-0410-b5e6-96231b3b80d8