Avoid unnecessary basic blocks for allocas, return and else

[dotdash]

These commits remove a bunch of empty or otherwise unnecessary blocks, reducing the size of the pre-optimization IR and improving its readability. librustc.ll created with --passes "" shrinks by about 120k lines which equals about 5% of the total size.

[dotdash]

This currently results in worse optimized code :-/

[pcwalton]

How much worse? I'm inclined to merge it anyway if it's fine.

[graydon]

Likewise, curious how much worse we're talking. Does it compile any faster on O0?

[dotdash]

Up to about 7% slower in some cases. Looks like it's due to the fact that the final function block doesn't reuse the "return" cleanup path anymore. Trying to fix that now.

[dotdash]

The new version shows no visible performance regression with -Z time-passes anymore and a cursory scan of a diff of the optimized librustc.ll from before and after the changes revealed only changes of label names. Should be good now.

[dotdash]

Build times for unoptimized libstd build with an unoptimized rustc:

Before

time: 2.736 s   parsing
time: 0.233 s   configuration 1
time: 2.903 s   expansion
time: 0.364 s   configuration 2
time: 0.271 s   maybe building test harness
time: 0.000 s   extra injection
time: 0.254 s   ast indexing
time: 0.054 s   external crate/lib resolution
time: 0.055 s   language item collection
time: 1.146 s   resolution
time: 0.000 s   looking for entry point
time: 0.138 s   freevar finding
time: 0.181 s   region resolution
time: 0.069 s   region parameterization inference
time: 0.486 s   type collecting
time: 0.566 s   coherence checking
time: 7.525 s   type checking
time: 0.225 s   const marking
time: 0.052 s   const checking
time: 0.121 s   privacy checking
time: 0.063 s   effect checking
time: 0.039 s   loop checking
time: 0.412 s   compute moves
time: 0.096 s   match checking
time: 0.338 s   liveness checking
time: 1.395 s   borrow checking
time: 0.184 s   kind checking
time: 0.125 s   reachability checking
time: 0.565 s   lint checking
time: 14.583 s  translation
time: 4.616 s   LLVM passes
time: 0.101 s   linking

After

time: 2.549 s   parsing
time: 0.215 s   configuration 1
time: 2.738 s   expansion
time: 0.340 s   configuration 2
time: 0.252 s   maybe building test harness
time: 0.000 s   extra injection
time: 0.240 s   ast indexing
time: 0.048 s   external crate/lib resolution
time: 0.049 s   language item collection
time: 1.044 s   resolution
time: 0.000 s   looking for entry point
time: 0.121 s   freevar finding
time: 0.168 s   region resolution
time: 0.063 s   region parameterization inference
time: 0.453 s   type collecting
time: 0.530 s   coherence checking
time: 6.901 s   type checking
time: 0.212 s   const marking
time: 0.047 s   const checking
time: 0.110 s   privacy checking
time: 0.057 s   effect checking
time: 0.034 s   loop checking
time: 0.384 s   compute moves
time: 0.088 s   match checking
time: 0.311 s   liveness checking
time: 1.320 s   borrow checking
time: 0.168 s   kind checking
time: 0.117 s   reachability checking
time: 0.505 s   lint checking
time: 13.802 s  translation
time: 4.457 s   LLVM passes
time: 0.096 s   linking

[graydon]

Great!