MIRPrinter: Avoid assert() when printing empty INLINEASM strings.

CodeGen uses MO_ExternalSymbol to represent the inline assembly strings.
Empty strings for symbol names appear to be invalid. For now just
special case the output code to avoid hitting an `assert()` in
`printLLVMNameWithoutPrefix()`.

This fixes https://llvm.org/PR33317

llvm-svn: 304815