cmd/compile: reduce slice growth in fuseBlockPlain

cmd/compile: reduce slice growth in fuseBlockPlain

Instead of always appending to c.Values,
choose whichever slice is larger;
b.Values will be set to nil anyway.

Appending once instead of in a loop also
limits slice growth to once per function call
and is more efficient.

Reduces max rss for the program in #18602 by 6.5%,
and eliminates fuseBlockPlain from the alloc_space
pprof output. fuseBlockPlain previously accounted
for 16.74% of allocated memory.

Updates #18602.

Change-Id: I417b03722d011a59a679157da43dc91f4425210e
Reviewed-on: https://go-review.googlesource.com/35114
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>