[X86] Add load folding test cases for the addcarryx intrinsic.

We are currently only able to fold a load in operand 1 to ADCX. A load in operand 0 will use the legacy ADC instruction.

Ultimately I want to remove isel support for ADCX, but first I'm going to fix the shortcomings I know of so I can write proper MIR tests to maintain coverage later.

llvm-svn: 341744