cmd/compile: fix panic-okay-to-inline change; adjust tests

This line of the inlining tuning experiment
https://go-review.googlesource.com/c/go/+/109918/1/src/cmd/compile/internal/gc/inl.go#347
was incorrectly rewritten in a later patch to use the call
cost, not the panic cost, and thus the inlining of panic
didn't occur when it should.  I discovered this when I
realized that tests should have failed, but didn't.

Fix is to make the correct change, and also to modify the
tests that this causes to fail.  One test now asserts the
new normal, the other calls "ppanic" instead which is
designed to behave like panic but not be inlined.

Change-Id: I423bb7f08bd66a70d999826dd9b87027abf34cdf
Reviewed-on: https://go-review.googlesource.com/116656
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/runtime-gdb_test.go

@@ -538,7 +538,7 @@ func TestGdbPanic(t *testing.T) {
 		`main`,
 	}
 	for _, name := range bt {
-		s := fmt.Sprintf("#.* .* in main\\.%v", name)
+		s := fmt.Sprintf("(#.* .* in )?main\\.%v", name)
 		re := regexp.MustCompile(s)
 		if found := re.Find(got) != nil; !found {
 			t.Errorf("could not find '%v' in backtrace", s)