cmd/compile: fix handling of ir.CurFunc during stenciling

The transform functions (specifically transformArgs, which is used from
transformCall/transformReturn) require that ir.CurFunc is set correctly.
Since transformCall() is used on the call of an instantiated generic
function, we need to set ir.CurFunc correctly in stencil(). Also,
correctly save/restore ir.CurFunc in genericSubst().

Without this fix, ir.CurFunc can be nil when we call TransformCall()
from stencil(), which leads to some temp variables being added
incorrectly to ir.TodoFunc (which leads to the fatal panic in the
issue).

Fixes #45722

Change-Id: Iddf4a67d28f2100dde8cde5dbc9ca1e00dad6089
Reviewed-on: https://go-review.googlesource.com/c/go/+/313869


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>