llvm-mc: A few more parsing / match tweaks.

 - Operands which are just a label should be parsed as immediates, not memory
   operands (from the assembler perspective).

 - Match a few more flavors of immediates.

 - Distinguish match functions for memory operands which don't take a segment
   register.

 - We match the .s for "hello world" now!

llvm-svn: 77745