Merge basic blocks where possible when generating LLVM IR.

In `codegen_assert_terminator` we decide if a BB's successor is a
candidate for merging, which requires that it be the only successor, and
that it only have one predecessor. That result then gets passed down,
and if it reaches `funclet_br` with the appropriate BB characteristics,
then no `br` instruction is issued, a `MergingSucc::True` result is
passed back, and the merging proceeds in `codegen_block`.

The commit also adds `CachedLlbb`, a new type to help keep track of
each BB that has been merged into its predecessor.

Author: nnethercote

compiler/rustc_codegen_ssa/src/lib.rs

@@ -1,12 +1,13 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
-#![feature(box_patterns)]
-#![feature(try_blocks)]
-#![feature(once_cell)]
 #![feature(associated_type_bounds)]
-#![feature(strict_provenance)]
-#![feature(int_roundings)]
+#![feature(box_patterns)]
 #![feature(if_let_guard)]
+#![feature(int_roundings)]
+#![feature(let_chains)]
 #![feature(never_type)]
+#![feature(once_cell)]
+#![feature(strict_provenance)]
+#![feature(try_blocks)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]