[X86] Use setcc ISD opcode for AVX512 integer comparisons all the way to isel

I don't believe there is any real reason to have separate X86 specific opcodes for vector compares. Setcc has the same behavior just uses a different encoding for the condition code.

I had to change the CondCodeAction for SETLT and SETLE to prevent some transforms from changing SETGT lowering.

Differential Revision: https://reviews.llvm.org/D43608

llvm-svn: 335173