cmd/compile: catch and report nowritebarrier violations later

Prior to this CL, the SSA backend reported violations
of the //go:nowritebarrier annotation immediately.
This necessitated emitting errors during SSA compilation,
which is not compatible with a concurrent backend.

Instead, check for such violations later.
We already save the data required to do a late check
for violations of the //go:nowritebarrierrec annotation.
Use the same data, and check //go:nowritebarrier at the same time.

One downside to doing this is that now only a single
violation will be reported per function.
Given that this is for the runtime only,
and violations are rare, this seems an acceptable cost.

While we are here, remove several 'nerrors != 0' checks
that are rendered pointless.

Updates #15756
Fixes #19250 (as much as it ever can be)

Change-Id: Ia44c4ad5b6fd6f804d9f88d9571cec8d23665cb3
Reviewed-on: https://go-review.googlesource.com/38973
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/ssa/func.go

@@ -37,7 +37,6 @@ type Func struct {
 	scheduled bool // Values in Blocks are in final order
 	NoSplit   bool // true if function is marked as nosplit.  Used by schedule check pass.
 
-	NoWB  bool     // write barrier is not allowed
 	WBPos src.XPos // line number of first write barrier
 
 	// when register allocation is done, maps value ids to locations