[dev.ssa] cmd/compile/internal/ssa: fix sign extension + load combo

[dev.ssa] cmd/compile/internal/ssa: fix sign extension + load combo

Load-and-sign-extend opcodes were being generated in the
wrong block, leading to having more than one memory variable
live at once.  Fix the rules + add a test.

Change-Id: Iadf80e55ea901549c15c628ae295c2d0f1f64525
Reviewed-on: https://go-review.googlesource.com/14591
Reviewed-by: Todd Neal <todd@tneal.org>
Run-TryBot: Todd Neal <todd@tneal.org>