cmd/compile: use cache in front of convI2I

cmd/compile: use cache in front of convI2I

This is the last of the getitab users to receive a cache.
We should now no longer see getitab (and callees) in profiles.
Hopefully.

Change-Id: I2ed72b9943095bbe8067c805da7f08e00706c98c
Reviewed-on: https://go-review.googlesource.com/c/go/+/531055
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>