[InstCombine] narrow icmp with extended operands of different widths

An intermediate extend is used to widen the narrow operand to the width of
the other (wider) operand. At that point, we have the same logic as the
existing transform that was restricted to folds of equal width zext/sext.

This mostly solves PR42700:
https://bugs.llvm.org/show_bug.cgi?id=42700

llvm-svn: 369519