Don't always run add_call_guards pass

[jonas-schievink]

It is only needed when add_retag runs.

(the pass is run again to split critical edges before codegen, that one is required)

[rust-highfive]

r? @petrochenkov

(rust_highfive has picked a reviewer for you, use r? to override)

[jonas-schievink]

@bors rollup=never

to capture perf impact when landing

[petrochenkov]

r? @ecstatic-morse

[ecstatic-morse]

This means that the MIR dump for add_retag will also have the effects of add_call_guards. I suspect this is fine, but just in case:

r? @RalfJung

Alternatively, you can add a variant/flag to AddCallGuards that will only run the pass if stacked borrows is enabled, thus preserving the MIR dumps. I would r+ that version if you don't want to wait for Ralf's OK.

[RalfJung]

Yeah this is fine, as long as AddRetag can rely on there not being any critical edges. :)

@bors r=ecstatic-morse,RalfJung

[bors]

📌 Commit 8c50006 has been approved by ecstatic-morse,RalfJung

[Dylan-DPC-zz]

@bors p=1

[bors]

⌛ Testing commit 8c50006 with merge 351eefe...

[bors]

☀️ Test successful - checks-azure
Approved by: ecstatic-morse,RalfJung
Pushing 351eefe to master...