[llvm-dev] [RFC] Enabling opt's -enable-new-pm by default with ENABLE_NEW_EXPERIMENTAL_PASS_MANAGER

[Arthur Eubanks via llvm-dev]

With many of the new PM issues ironed out, I'd like to turn on opt's
-enable-new-pm flag by default when we build LLVM
with ENABLE_NEW_EXPERIMENTAL_PASS_MANAGER on.

-enable-new-pm essentially translates the legacy way of specifying opt
passes (e.g. -instcombine) into the new way of specifying passes (e.g.
-passes=instcombine), making tests that use `opt -instcombine` run under
the new PM, unless specifically opted out via `-enable-new-pm=0`.

One issue is that until we enable the new PM by default, testing locally
may not show issues that pop up until a bot
with ENABLE_NEW_EXPERIMENTAL_PASS_MANAGER on complains after committing,
since basically everybody has ENABLE_NEW_EXPERIMENTAL_PASS_MANAGER off
locally. Hopefully the amount of time between conditionally turning on
opt's -enable-new-pm and turning on the new PM by default everywhere is
fairly small and this won't be too big of an issue. I haven't seen this
discrepancy appear too often when looking at existing passing tests failing
when -enable-new-pm is turned on.
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