cmd/compile: resolve TODO in inl.go

If the condition is a bool constant, there's no need to walk both
branches.

Passes toolstash -cmp.

Change-Id: I4ee5e3553ce07c2213efba0d33d869b4a1b57783
Reviewed-on: https://go-review.googlesource.com/c/go/+/347911


Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>