cmd/compile: correct argument area size for typedmemmove/typedmemclr

When the write barrier pass emits typedmemmove/typedmemclr calls,
even the arguments are in registers, we still need to leave space
for the spill slots. Count that space. Otherwise when the callee
spills arguments it may clobber locals on the caller's frame.

Change-Id: I5326943427feaf66cab7658a5bef55b3baf5d345
Reviewed-on: https://go-review.googlesource.com/c/go/+/307824


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: David Chase <drchase@google.com>