[InstCombine] rename variables for readability; NFC

There's no reason to use "CI" (cast instruction) when
we know that the value is a more specific (exact) type
of instruction (although we might want to common-ize some
of this code to eliminate duplication or logic diffs).

It's also visually difficult to distinguish between "CI",
"ICI", and "IC" acronyms (and those could change meaning
depending on context).

This was partially changed in earlier commits, so this
makes this pair of functions consistent.