cmd/compile: ensure first instruction of function is stmt
cmd/compile: ensure first instruction of function is stmt

In gdb, "b f" gets confused if the first instruction of "f"
is not marked as a statement in the DWARF line table.

To ensure gdb is not confused, move the first statement
marker in "f" to its first instruction.

The screwy-looking conditional for "what's the first
instruction with a statement marker" will become simpler in
the future.

Fixes #24695.

Change-Id: I2eef81676b64d1bd9bff5da03b89b9dc0c18f44f
Reviewed-on: https://go-review.googlesource.com/104955
Run-TryBot: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

Cofyc/go,josharian/go,akutz/go,CAFxX/go,golang/go,CAFxX/go,Cofyc/go,akutz/go,josharian/go,CAFxX/go,Cofyc/go,josharian/go,akutz/go,akutz/go,cloudflare/go,nawawi/go,cloudflare/go,josharian/go,cloudflare/go,CAFxX/go,akutz/go,golang/go,nawawi/go,akutz/go,Cofyc/go,golang/go,josharian/go,Cofyc/go,golang/go,Cofyc/go,CAFxX/go,nawawi/go,akutz/go,golang/go,akutz/go,nawawi/go,Cofyc/go,cloudflare/go,nawawi/go,Cofyc/go,cloudflare/go,josharian/go,josharian/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,josharian/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,golang/go