[X86] Simplify the interface to getCondNoFromDesc.

Instead of taking a SkipDefs parameter, rename to getCondSrcNoFromDesc
and have it return the source operand number. Make getCondFromMI
responsible for adding the number of Defs for MI instructions.

While there remove some unneeded casts to unsigned and check for
negative numbers instead of explicitly -1. Less than 0 is easier
for a compiler to codegen.

Differential Revision: https://reviews.llvm.org/D122113