[InstCombine] Missed optimization: fold select if the user of its select user indicates the condition

[XChy]

Alive2 proof: https://alive2.llvm.org/ce/z/EDjfon

Motivating example

define i32 @src(i32 %a, i1 %c1, i1 %c2){
entry:
  %s1 = select i1 %c1, i32 23, i32 45
  %s2 = select i1 %c2, i32 666, i32 %s1
  %s3 = select i1 %c1, i32 789, i32 %s2
  ret i32 %s3
}

can be folded to:

define i32 @tgt(i32 %a, i1 %c1, i1 %c2){
entry:
  %s2 = select i1 %c2, i32 666, i32 45
  %s3 = select i1 %c1, i32 789, i32 %s2
  ret i32 %s3
}

Real-world motivation

This snippet of IR is derived from libdeflate/lib/crc32.c@dispatch_crc32 (after O3 pipeline, original IR comes from llvm-opt-benchmark).
The example above is a reduced version. If you're interested in the original suboptimal IR and optimal IR, see also:https://godbolt.org/z/rjf9zWM81

Let me know if you can confirm that it's an optimization opportunity, thanks.

[vfdff]

should also support float type, https://alive2.llvm.org/ce/z/3Uaiis