The patch adds a possibility to make library calls.
An important thing about library functions - they must be defined within the current module. This basically should guarantee that we produce a valid PTX assembly (without calls to not defined functions). The one who wants to use the libcalls is probably will have to link against compiler-rt or any other implementation.
Currently, it's completely impossible to make library calls because of error LLVM ERROR: Cannot select: i32 = ExternalSymbol '...'. But we can lower ExternalSymbol to TargetExternalSymbol and verify if the function definition is available.
Also, there was an issue with a DAG during legalisation. When we expand instruction into libcall, the inner call-chain isn't being "integrated" into outer chain. Since the last "data-flow" (call retval load) node is located in call-chain earlier than CALLSEQ_END node, the latter becomes a leaf and therefore a dead node (and is being removed quite fast). Proposed here solution relies on another data-flow pseudo nodes (ProxyReg) which purpose is only to keep CALLSEQ_END at legalisation and instruction selection phases - we remove the pseudo instructions before register scheduling phase.