cmd/compile: optimize unified IR handling of imported functions

This CL skips reading of imported function bodies except from the
local package or for inlining. Previously, we wasted a bunch of CPU
and memory by reading in normal function bodies for all imported
functions and methods.

Change-Id: I0b3df3f4a3e2819117dac11a1f9e4526288c14d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/347030
Trust: Matthew Dempsky <[email protected]>
Run-TryBot: Matthew Dempsky <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Cuong Manh Le <[email protected]>

Author: mdempsky

src/cmd/compile/internal/noder/reader.go

@@ -920,6 +920,12 @@ func (r *reader) addBody(fn *ir.Func) {
 	pri := pkgReaderIndex{r.p, r.reloc(relocBody), r.dict}
 	bodyReader[fn] = pri
 
+	if fn.Nname.Defn == nil {
+		// Don't read in function body for imported functions.
+		// See comment in funcExt.
+		return
+	}
+
 	if r.curfn == nil {
 		todoBodies = append(todoBodies, fn)
 		return