RFR: 8257190: simplify PhaseIdealLoop constructors

[Xin Liu]

On Fri, 27 Nov 2020 00:07:53 GMT, Xin Liu <xliu at openjdk.org> wrote:

> 8257190: simplify PhaseIdealLoop constructors

a. PhaseIdealLoop( PhaseIterGVN &igvn)
b. PhaseIdealLoop(PhaseIterGVN &igvn, const PhaseIdealLoop *verify_me)
c. PhaseIdealLoop(PhaseIterGVN &igvn, LoopOptsMode mode)

I propose 3 changes to simplify them.
1. add assertion in the constructor c. C2 shouldn't use mode = LoopOptsVerify for it.
2. merge a and b into one constructor.
3. make the merged verification ctor only for debug builds.

-------------

PR: https://git.openjdk.java.net/jdk/pull/1473