MaybeUninit::assume_init_read should have noundef load metadata

I was looking into `array::IntoIter` optimization, and noticed that it wasn't annotating the loads with `noundef` for simple things like `array::IntoIter<i32, N>`.

Turned out to be a more general problem as `MaybeUninit::assume_init_read` isn't marking the load as initialized (<https://rust.godbolt.org/z/Mxd8TPTnv>), which is unfortunate since that's basically its reason to exist.

This PR lowers `ptr::read(p)` to `copy *p` in MIR, which fortuitiously also improves the IR we give to LLVM for things like `mem::replace`.

compiler/rustc_span/src/symbol.rs

@@ -1153,6 +1153,7 @@ symbols! {
         read_enum_variant_arg,
         read_struct,
         read_struct_field,
+        read_via_copy,
         readonly,
         realloc,
         reason,