[dev.ssa] cmd/compile: Use varkill only for non-SSAable vars

[dev.ssa] cmd/compile: Use varkill only for non-SSAable vars

For variables which get SSA'd, SSA keeps track of all the def/kill.
It is only for on-stack variables that we need them.

This reduces stack frame sizes significantly because often the
only use of a variable was a varkill, and without that last use
the variable doesn't get allocated in the frame at all.

Fixes #12602

Change-Id: I3f00a768aa5ddd8d7772f375b25f846086a3e689
Reviewed-on: https://go-review.googlesource.com/14758
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>