[TargetLowering] Extract simplifySetCCs ctpop into a separate function. NFCI

As requested in D89346. This allows us to add some early outs.

I reordered some checks a little bit to make the more common bail outs happen earlier. Like checking opcode before checking hasOneUse. And I moved the bit width check to make sure it was safe to look through a truncate to the spot where we look through truncates instead of after.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D89494