[dev.ssa] cmd/compile/internal/ssa: distinguish exit and return blocks
It is confusing to have exceptional edges jump back into
real code. Distinguish return blocks, which execute acutal
code, and the exit block, which is a merge point for the regular
and exceptional return flow.
Prevent critical edge insertion from adding blocks on edges
into the exit block. These added blocks serve no purpose and
add a bunch of dead jumps to the assembly output. Furthermore,
live variable analysis is confused by these jumps.
Change-Id: Ifd69e6c00e90338ed147e7cb351b5100dc0364df
Reviewed-on: https://go-review.googlesource.com/14254 Reviewed-by: David Chase <drchase@google.com>