cmd/compile: refactor inlining parameters; inline panic

Inlining was refactored to perform tuning experiments,
with the "knobs" now set to also inline functions/methods
that include panic(), and -l=4 (inline calls) now expressed
as a change to costs, rather than scattered if-thens.

The -l=4 inline-calls penalty is chosen to be the best
found during experiments; it makes some programs much
larger and slower (notably, the compiler itself) and is
believed to be risky for machine-generated code in general,
which is why it is not the default.  It is also not
well-tested with the debugger and DWARF output.

This change includes an explicit go:noinline applied to the
method that is the largest cause of compiler binary growth
and slowdown for midstack inlining; there are others,
ideally whatever heuristic eventually appears will make
this unnecessary.

Change-Id: Idf7056ed2f961472cf49d2fd154ee98bef9421e2
Reviewed-on: https://go-review.googlesource.com/109918


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>