runtime: make _TinySizeClass an int8 to prevent use as spanClass

Currently _TinySizeClass is untyped, which means it can accidentally
be used as a spanClass (not that I would know this from experience or
anything). Make it an int8 to avoid this mix up.

This is a cherry-pick of dev.garbage commit 81b74bf9c5.

Change-Id: I1e69eccee436ea5aa45e9a9828a013e369e03f1a
Reviewed-on: https://go-review.googlesource.com/41254
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/malloc.go

@@ -111,7 +111,7 @@ const (
 
 	// Tiny allocator parameters, see "Tiny allocator" comment in malloc.go.
 	_TinySize      = 16
-	_TinySizeClass = 2
+	_TinySizeClass = int8(2)
 
 	_FixAllocChunk  = 16 << 10               // Chunk size for FixAlloc
 	_MaxMHeapList   = 1 << (20 - _PageShift) // Maximum page length for fixed-size list in MHeap.

src/runtime/mheap.go

@@ -326,7 +326,7 @@ type spanClass uint8
 
 const (
 	numSpanClasses = _NumSizeClasses << 1
-	tinySpanClass  = tinySizeClass<<1 | 1
+	tinySpanClass  = spanClass(tinySizeClass<<1 | 1)
 )
 
 func makeSpanClass(sizeclass uint8, noscan bool) spanClass {