cmd/compile: delay expansion of OpArg until expand_calls

As it says, delay expanpsion of OpArg to the expand_calls phase,
to enable (eventually) interprocedural SSA optimizations, and
(sooner) change to a register ABI.

Includes a round of cleanup to function names and comments,
largely to match the expanded scope of the functions.

This CL removes the per-function dependence on GOSSAHASH,
but the go116lateCallExpansion kill switch remains (and was
tested locally to ensure it worked).

Two functions in expand_calls.go that performed overlapping
things were combined into a single function that is called
twice.

Fixes #42236.
For #40724.

Change-Id: Icbb78947eaa39f17f2c1210d5c2caef20abd6571
Reviewed-on: https://go-review.googlesource.com/c/go/+/262117
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/fmtmap_test.go

@@ -136,7 +136,6 @@ var knownFormats = map[string]string{
 	"cmd/compile/internal/types.EType %s":             "",
 	"cmd/compile/internal/types.EType %v":             "",
 	"cmd/internal/obj.ABI %v":                         "",
-	"cmd/internal/src.XPos %v":                        "",
 	"error %v":                                        "",
 	"float64 %.2f":                                    "",
 	"float64 %.3f":                                    "",