[MCDwarf] Do not emit useless line table opcode.

No need to emit a DW_LNS_advance_pc with a 0 increment. Found out while
comparing dsymutil's and LLVM's line table encoding. Not a correctenss
fix, just a small encoding size optimization.

I'm not sure how to generate a sequence that triggers this, and moreover
llvm-dwardump doesn't dump the line table program, thus the effort
involved in creating a testcase for this trivial patch seemed out of
proportion.

llvm-svn: 232332