[dev.regabi] cmd/compile: convert OPANIC argument to interface{} during typecheck

Currently, typecheck leaves arguments to OPANIC as their original
type. This CL changes it to insert implicit OCONVIFACE operations to
convert arguments to `interface{}` like how any other function call
would be handled.

No immediate benefits, other than getting to remove a tiny bit of
special-case logic in order.go's handling of OPANICs. Instead, the
generic code path for handling OCONVIFACE is used, if necessary.
Longer term, this should be marginally helpful for #43753, as it
reduces the number of cases where we need values to be addressable for
runtime calls.

However, this does require adding some hacks to appease existing
tests:

1. We need yet another kludge in inline budgeting, to ensure that
reflect.flag.mustBe stays inlinable for cmd/compile/internal/test's
TestIntendedInlining.

2. Since the OCONVIFACE expressions are now being introduced during
typecheck, they're now visible to escape analysis. So expressions like
"panic(1)" are now seen as "panic(interface{}(1))", and escape
analysis warns that the "interface{}(1)" escapes to the heap. These
have always escaped to heap, just now we're accurately reporting about
it.

(Also, unfortunately fmt.go hides implicit conversions by default in
diagnostics messages, so instead of reporting "interface{}(1) escapes
to heap", it actually reports "1 escapes to heap", which is
confusing. However, this confusing messaging also isn't new.)

Change-Id: Icedf60e1d2e464e219441b8d1233a313770272af
Reviewed-on: https://go-review.googlesource.com/c/go/+/284412
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Matthew Dempsky <mdempsky@google.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/test/escape2n.go b/test/escape2n.go
index 46e58f8566..7c8208aa73 100644
--- a/test/escape2n.go
+++ b/test/escape2n.go
@@ -1547,7 +1547,7 @@ func foo153(v interface{}) *int { // ERROR "v does not escape"
 	case int: // ERROR "moved to heap: x$"
 		return &x
 	}
-	panic(0)
+	panic(0) // ERROR "0 escapes to heap"
 }
 
 // issue 8185 - &result escaping into result