cmd/compile: generalize racewalk to instrument (naming change)

cmd/compile: generalize racewalk to instrument (naming change)

This is mechanical change that is a step toward reusing the racewalk
pass for a more general instrumentation pass.  The first use will be to
add support for the memory sanitizer.

Change-Id: I75b93b814ac60c1db1660e0b9a9a7d7977d86939
Reviewed-on: https://go-review.googlesource.com/16105
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>