runtime: update mkmalloc to make generated code look nicer

runtime: update mkmalloc to make generated code look nicer

This cl adds a new operation that can remove an if statement or replace
it with its body if its condition is var or !var for some variable var
that's being replaced with a constant.

Change-Id: I864abf1f023b2a66b2299ca65d4f837d6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/724940
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Michael Matloob <matloob@google.com>