Fix a logic bug in x86 vector codegen: sext (zext (x) ) != sext (x) (PR20472).

Remove a block of code from LowerSIGN_EXTEND_INREG() that was added with:
http://llvm.org/viewvc/llvm-project?view=revision&revision=177421

And caused:
http://llvm.org/bugs/show_bug.cgi?id=20472 (more analysis here)
http://llvm.org/bugs/show_bug.cgi?id=18054

The testcases confirm that we (1) don't remove a zext op that is necessary and (2) generate
a pmovz instead of punpck if SSE4.1 is available. Although pmovz is 1 byte longer, it allows
folding of the load, and so saves 3 bytes overall.

Differential Revision: http://reviews.llvm.org/D4909