Stop turning transmutes into discriminant reads in mir-optPartially revertsrust-lang#109612, as afterrust-lang#109993 these aren't actually equivalent any more, and I'm no longer confident this was ever an improvement in the first place.Having this "simplification" meant that similar-looking code actually did somewhat different things.  For example,```rustpub unsafe fn demo1(x: std::cmp::Ordering) -> u8 {    std::mem::transmute(x)}pub unsafe fn demo2(x: std::cmp::Ordering) -> i8 {    std::mem::transmute(x)}```in nightly today is generating <https://rust.godbolt.org/z/dPK58zW18>```llvmdefine noundef i8 `@_ZN7example5demo117h341ef313673d2ee6E(i8` noundef %x) unnamed_addr #0 {  %0 = icmp uge i8 %x, -1  %1 = icmp ule i8 %x, 1  %2 = or i1 %0, %1  call void `@llvm.assume(i1` %2)  ret i8 %x}define noundef i8 `@_ZN7example5demo217h5ad29f361a3f5700E(i8` noundef %0) unnamed_addr #0 {  %x = alloca i8, align 1  store i8 %0, ptr %x, align 1  %1 = load i8, ptr %x, align 1, !range !2, !noundef !3  ret i8 %1}```Which feels too different when the original code is essentially identical.---Aside: that example is different *after* optimizations too:```llvmdefine noundef i8 `@_ZN7example5demo117h341ef313673d2ee6E(i8` noundef returned %x) unnamed_addr #0 {  %0 = add i8 %x, 1  %1 = icmp ult i8 %0, 3  tail call void `@llvm.assume(i1` %1)  ret i8 %x}define noundef i8 `@_ZN7example5demo217h5ad29f361a3f5700E(i8` noundef returned %0) unnamed_addr#1 {  ret i8 %0}```so turning the `Transmute` into a `Discriminant` was arguably just making things worse, so leaving it alone instead -- and thus having less code in rustc -- seems clearly better.