This patch adds the aforementioned fold to InstCombine. This pattern is
produced after naive implementations of 3-way comparison in high-level
languages are transformed into LLVM IR and then optimized.
Proofs: https://alive2.llvm.org/ce/z/w4QLq_
When converting a switch with two cases and a default into a
select, also handle the denegerate case where two cases have the
same value.
Generate this case directly as
%or = or i1 %cmp1, %cmp2
%res = select i1 %or, i32 %val, i32 %default
rather than
%sel1 = select i1 %cmp1, i32 %val, i32 %default
%res = select i1 %cmp2, i32 %val, i32 %sel1
as InstCombine is going to canonicalize to the former anyway.
We handle the case where we have two cases and a default all having
different values, but not the case where two cases happen to have
the same one.
The PhaseOrdering test is a particularly bad example where this
showed up.
