[X86] Fix checked arithmetic for i8 on X86.

When lowering a ISD::BRCOND into a test+branch, make sure that we
always use the correct condition code to emit the test operation.

This fixes PR19858: "i8 checked mul is wrong on x86".

Patch by Keno Fisher!



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210032 91177308-0d34-0410-b5e6-96231b3b80d8