[X86] Narrow i64 compares with constant to i32 when the upper 32-bits are known zero.

This catches some cases. There are probably ways to improve this.
I tried doing it as a combine on the setcc, but that broke
some cases involving flag reuse in place of test.

I renamed the isX86CCUnsigned to isX86CCSigned and flipped its
polarity to make it consistent with the similar functions for
ISD::SETCC. This avoids calling EQ/NE as being signed or unsigned.

Fixes PR43823.

Differential Revision: https://reviews.llvm.org/D69499