[X86] Make the subus special case in LowerVSETCC self contained

Previously this code overrode the flags and opcode used by the later code in LowerVSETCC. This makes the code difficult to read and follow.

This patch moves all the SUBUS code into its own function and makes it responsible for creating its own SDNodes on success.

Differential Revision: https://reviews.llvm.org/D43530

llvm-svn: 325827