[TargetLowering] Simplify some code in SimplifySetCC that tries to handle...

[TargetLowering] Simplify some code in SimplifySetCC that tries to handle SIGN_EXTEND_INREG operand types that should never happen. NFCI

There was code to handle the first operand being different than
the result type. And code to handle first operand having the
same type as the type to extend from. This should never happen
for a correctly formed SIGN_EXTEND_INREG. I've replace the
code with asserts.

I also noticed we created the same APInt twice so I've reused it.