cmd/compile: make sure instrumented call has type width

The width of the type of an external variable defined with a type
literal may not be set when the instrumentation pass is run.  There are
two cases in the standard library that fail without the call to dowidth:

../../../src/encoding/base32/base32.go:322: constant -1000000000 overflows uintptr
../../../src/encoding/base32/base32.go:329: constant -1000000000 overflows uintptr
../../../src/encoding/json/encode.go:385: constant -1000000000 overflows uintptr
../../../src/encoding/json/encode.go:387: constant -1000000000 overflows uintptr

Change-Id: I7c3334f7decdb7488595ffe4090cd262d7334283
Reviewed-on: https://go-review.googlesource.com/16331


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>