runtime: allow inlining of stackmapdata

Also do very minor code cleanup.

name                old time/op  new time/op  delta
StackCopyPtr-8      84.8ms ± 6%  82.9ms ± 5%  -2.19%  (p=0.000 n=95+94)
StackCopy-8         68.4ms ± 5%  65.3ms ± 4%  -4.54%  (p=0.000 n=99+99)
StackCopyNoCache-8   107ms ± 2%   105ms ± 2%  -2.13%  (p=0.000 n=91+95)

Change-Id: I2d85ede48bffada9584d437a08a82212c0da6d00
Reviewed-on: https://go-review.googlesource.com/109001
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

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

@@ -60,6 +60,7 @@ func TestIntendedInlining(t *testing.T) {
 			"releasem",
 			"round",
 			"roundupsize",
+			"stackmapdata",
 			"stringStructOf",
 			"subtract1",
 			"subtractb",

src/runtime/symtab.go

@@ -916,10 +916,13 @@ type stackmap struct {
 
 //go:nowritebarrier
 func stackmapdata(stkmap *stackmap, n int32) bitvector {
-	if n < 0 || n >= stkmap.n {
+	// Check this invariant only when stackDebug is on at all.
+	// The invariant is already checked by many of stackmapdata's callers,
+	// and disabling it by default allows stackmapdata to be inlined.
+	if stackDebug > 0 && (n < 0 || n >= stkmap.n) {
 		throw("stackmapdata: index out of range")
 	}
-	return bitvector{stkmap.nbit, (*byte)(add(unsafe.Pointer(&stkmap.bytedata), uintptr(n*((stkmap.nbit+7)>>3))))}
+	return bitvector{stkmap.nbit, addb(&stkmap.bytedata[0], uintptr(n*((stkmap.nbit+7)>>3)))}
 }
 
 // inlinedCall is the encoding of entries in the FUNCDATA_InlTree table.