[FastISel][X86] Fold XALU condition into branch and compare.

Optimize the codegen of select and branch instructions to directly use the
EFLAGS from the {s|u}{add|sub|mul}.with.overflow intrinsics.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211645 91177308-0d34-0410-b5e6-96231b3b80d8