Different ways of writing Option::or codegen differently

[scottmcm]

Copying this here from #101210 (comment) cc @veber-alex

Demo: https://rust.godbolt.org/z/96W4h87Yn

The currently-optimized form isn't allowed to be transformed into the better one: https://alive2.llvm.org/ce/z/DXXRPX

A couple thoughts on things that might help in the codegen:

• The parameters should be marked noundef (this will hopefully be addressed by Array-as-Immediate array ABI is missing noundef parameter metadata #123183 )
• While we do seem to emit the !range metadata (https://rust.godbolt.org/z/KKTKTa1qs), we're hitting the classic problem of us emitting

  %4 = load i8, ptr %opta, align 1, !range !3, !noundef !4
  %5 = trunc i8 %4 to i1
  %_3 = zext i1 %5 to i64
  switch i64 %_3, label %bb1 [
    i64 0, label %bb2
    i64 1, label %bb3
  ]

And LLVM forgetting both different ways that we told it that inputs like 0x00_05 (the one Alive found) are UB.

Maybe us emitting trunc nuw will be the eventual solution here? If the trunc nuw+zext could fold to just zext rather than and +zext maybe it'd be enough...

---

Adding some assumes does give LLVM enough information to be allowed to optimize again (https://alive2.llvm.org/ce/z/e6NWxA) though seemingly it doesn't from that starting point

  %discr0 = and i16 %0, 255
  %discr1 = and i16 %1, 255
  %discr0bound = icmp ult i16 %discr0, 2
  %discr1bound = icmp ult i16 %discr1, 2
  call void @llvm.assume(i1 %discr0bound)
  call void @llvm.assume(i1 %discr1bound)