cmd/compile: add sources for inlined functions to ssa.html

This CL adds the source code of all inlined functions
into the function specified in $GOSSAFUNC.
The code is appended to the sources column of ssa.html.

ssaDumpInlined is populated with references to inlined functions.
Then it is used for dumping the sources in buildssa.

The source columns contains code in following order:
target function, inlined functions sorted by filename, lineno.

Fixes #25904

Change-Id: I4f6d4834376f1efdfda1f968a5335c0543ed36bc
Reviewed-on: https://go-review.googlesource.com/126606


Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>