compiler: scan all function literals for escape analysis

compiler: scan all function literals for escape analysis

We were scanning only function literals with closures, but not all
function literals have closures.

Discovered because compiler failed building 1.15rc1, as there is a
function literal in the runtime package (p1 in hexdumpWords) that has
no closure and, without escape analysis, was forcing a variable to the
heap which is not permitted in the runtime.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/244802