cmd/compile: fix uninitialized memory during type switch assertE2I2

Fixes arm64 builder crash.

The bug is possible on all architectures; you just have to get lucky
and hit a preemption or a stack growth on entry to assertE2I2.
The test stacks the deck.

Change-Id: I8419da909b06249b1ad15830cbb64e386b6aa5f6
Reviewed-on: https://go-review.googlesource.com/12890


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>