[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
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@256624 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm