tree f727590a63a1676289f8695d50343e175ed4d157
parent 739805457ff7910fc880f328ec383473b8627a1a
author Cherry Zhang <cherryyz@google.com> 1545324969 -0500
committer Cherry Zhang <cherryyz@google.com> 1545446068 +0000

passes: improve degenerate Phi detection

It is possible that a group of mutually dependent Phis have a
constant value, like
	a = phi(null, b)
and in a different block
	b = phi(a, null)

The simple algorithm could not detect it.

The codegen preparation pass may reduce this to the constant,
causing problem if it is recorded in the stack map.

Improve degenerate Phi detection to handle this case.

Change-Id: Ie81a78796cfd0f9920411af18758343d886a55e3
Reviewed-on: https://go-review.googlesource.com/c/155497
Reviewed-by: Than McIntosh <thanm@google.com>