compiler: support inlining functions with if statements

This increases the number of inlinable functions from 455 to 500.
An example of a newly inlinable function is strings.Compare.

Change-Id: I3e672e792e9d3283ff255044af430d982a7e46ca
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/181137
Reviewed-by: Than McIntosh <thanm@google.com>
3 files changed