[X86] Fix two places that appear to misuse peekThroughOneUseBitcasts

peekThroughOneUseBitcasts checks the use count of the operand of the bitcast. Not the bitcast itself. So I think that means we need to do any outside haseOneUse checks before calling the function not after.

I was working on another patch where I misused the function and did a very quick audit to see if I there were other similar mistakes.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D83598