[release-branch.go1.5] cmd/compile: fix crash with -race on large expr...

[release-branch.go1.5] cmd/compile: fix crash with -race on large expr containing string->[]byte conversion

The assumption is that there are no nested function calls in complex expressions.
For the most part that assumption is true. It wasn't for these calls inserted during walk.
Fix that.

I looked through all the calls to mkcall in walk and these were the only cases
that emitted calls, that could be part of larger expressions (like not delete),
and that were not already handled.

Fixes #12225.

Change-Id: Iad380683fe2e054d480e7ae4e8faf1078cdd744c
Reviewed-on: https://go-review.googlesource.com/17034


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-on: https://go-review.googlesource.com/17125