[dev.ssa] cmd/compile: fix registers for in-place instructions
Some of these were right; others weren't.
Fixes 'GOGC=off GOSSAPKG=mime go test -a mime'.
The right long term fix is probably to teach the
register allocator about in-place instructions.
In the meantime, all the tests that we can run
now pass.
Change-Id: I8e37b00a5f5e14f241b427d45d5f5cc1064883a2
Reviewed-on: https://go-review.googlesource.com/12664 Reviewed-by: Keith Randall <khr@golang.org>