Split Ty::is_empty method into is_never and is_uninhabited

This is simpler to work with than `with_cond`.

Factors out the common pattern across the several places that do
arithmetic checks

Add, Sub, Mul, Shl, and Shr are checked using a new Rvalue:
CheckedBinaryOp, while Div, Rem and Neg are handled with explicit checks
in the MIR.

Coercion casts (`expr as T` where the type of `expr` can be coerced to
`T`) are essentially no-ops, as the actual work is done by a coercion.
Previously a check for type equality was used to avoid emitting the
redundant cast in the MIR, but this failed for coercion casts of
function items that had lifetime parameters. The MIR trans code doesn't
handle `FnPtr -> FnPtr` casts and produced an error.

Also fixes a bug with type ascription expressions not having any
adjustments applied.

Fixes #33295

In code like `let x = y = z;`, `y = z` goes through `as_rvalue`, which
didn't handle it. Now it translates the assignment and produces `()`
directly.

Fixes #31504

This gets rid of Drop(Free, _) MIR construct by synthesizing a call to language item which
takes care of dropping instead.

Previously the code would fail to dereference the temporary.

This is done mostly so that we can refer to MIR types in csearch and other metadata related area.

the right information, but it's hard to maintain in the face of
optimizations, and in the form that the analyses probably actually want.

This commit contains some of the changes proposed by a rustfmt invocation,
chosen based on the fairly non-deterministic metric of how much I liked the
change. I expect we will run rustfmt on this crate again later, probably
accepting more of its changes. For now, this is already an improvement over
the status-quo.