MIR: Compute always live locals more efficiently

[pierwill]

See FIXME below:

rust/compiler/rustc_mir_dataflow/src/storage.rs

Lines 4 to 10 in 20dd693

	/// The set of locals in a MIR body that do not have `StorageLive`/`StorageDead` annotations.
	///
	/// These locals have fixed storage for the duration of the body.
	//
	// FIXME: Currently, we need to traverse the entire MIR to compute this. We should instead store it
	// as a field in the `LocalDecl` for each `Local`.
	pub fn always_live_locals(body: &mir::Body<'_>) -> BitSet<Local> {

@rustbot label +T-compiler +A-mir

[pierwill]

@rustbot claim

[pierwill]

@nikomatsakis and I discussed this PR and issue yesterday. The tradeoff involved is this: improving the performance of this for loop, versus complicating a common data structure--to help with this one case. In the absence of profiling data, it's not clear how valuable this work is. Especially, in my view, if there are outstanding questions about MIR semantics that this could complicate.

#99025 (comment)