[X86] Replace custom scalar integer absolute matching with ISD::ABS lowering.

x86 has its own copy of integer absolute pattern matching to combine directly to a SUB+CMOV.

This patch removes the x86 combine and adds custom lowering support for ISD::ABS instead, allowing us to use the DAGCombiner version.

Additional test cases are already covered by iabs.ll (rL315706 and rL315711).

Differential Revision: https://reviews.llvm.org/D38895

llvm-svn: 316162