[InstCombine] Fold comparison of integers by parts
Let's say you represent (i32, i32) as an i64 from which the parts
are extracted with lshr/trunc. Then, if you compare two tuples by
parts you get something like A == B && A == B, just
that the part extraction happens by lshr/trunc and not a narrow
load or similar.
The fold implemented here reduces such equality comparisons by
converting them into a comparison on a larger part of the integer
(which might be the whole integer). It handles both the "and of eq"
and the conjugated "or of ne" case.
I'm being conservative with one-use for now, though this could be
relaxed if profitable (the base pattern converts 11 instructions
into 5 instructions, but there's quite a few variations on how it
can play out).
Differential Revision: https://reviews.llvm.org/D101232