cmd/compile: reduce debugger-worsening line number churn

Reuse block head or preceding instruction's line number for
register allocator's spill, fill, copy, rematerialization
instructionsl; and also for phi, and for no-src-pos
instructions.  Assembler creates same line number tables
for copy-predecessor-line and for no-src-pos,
but copy-predecessor produces better-looking assembly
language output with -S and with GOSSAFUNC, and does not
require changes to tests of existing assembly language.

Split "copyInto" into two cases, one for register allocation,
one for otherwise.  This caused the test score line change
count to increase by one, which may reflect legitimately
useful information preserved.  Without any special treatment
for copyInto, the change count increases by 21 more, from
51 to 72 (i.e., quite a lot).

There is a test; using two naive "scores" for line number
churn, the old numbering is 2x or 4x worse.

Fixes #18902.

Change-Id: I0a0a69659d30ee4e5d10116a0dd2b8c5df8457b1
Reviewed-on: https://go-review.googlesource.com/36207


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>