cmd/compile: sort CFG blocks in PC order during liveness

cmd/compile: sort CFG blocks in PC order during liveness

This CL changes the order that liveness analysis visits CFG blocks to
PC order, rather than RPO. This doesn't meaningfully change anything
except that the PCDATA_StackMapIndex values will be assigned in PC
order too.

However, this does have the benefit that the subsequent CL to port
liveness analysis to the SSA CFG (which has blocks in PC order) will
now pass toolstash-check.

Change-Id: I1de5a2eecb8027723a6e422d46186d0c63d48c8d
Reviewed-on: https://go-review.googlesource.com/38086
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>