cmd/link: consider interface conversions only in reachable code

The linker prunes methods that are not directly reachable if the
receiver type is never converted to interface. Currently, this
"never" is too strong: it is invalidated even if the interface
conversion is in an unreachable function. This CL improves it by
only considering interface conversions in reachable code. To do
that, we introduce a marker relocation R_USEIFACE, which marks
the target symbol as UsedInIface if the source symbol is reached.

binary size    before      after
cmd/compile   18897528   18887400
cmd/go        13607372   13470652

Change-Id: I66c6b69eeff9ae02d84d2e6f2bc7f1b29dd53910
Reviewed-on: https://go-review.googlesource.com/c/go/+/256797
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/compile/internal/gc/pgen.go

@@ -231,6 +231,11 @@ func compile(fn *Node) {
 		return
 	}
 
+	// Set up the function's LSym early to avoid data races with the assemblers.
+	// Do this before walk, as walk needs the LSym to set attributes/relocations
+	// (e.g. in markTypeUsedInInterface).
+	fn.Func.initLSym(true)
+
 	walk(fn)
 	if nerrors != 0 {
 		return
@@ -250,9 +255,6 @@ func compile(fn *Node) {
 		return
 	}
 
-	// Set up the function's LSym early to avoid data races with the assemblers.
-	fn.Func.initLSym(true)
-
 	// Make sure type syms are declared for all types that might
 	// be types of stack objects. We need to do this here
 	// because symbols must be allocated before the parallel