runtime: speed up stack copying a little

Remove a branch and a stack spill.

name                old time/op  new time/op  delta
StackCopy-8         79.2ms ± 1%  79.1ms ± 2%    ~     (p=0.063 n=96+95)
StackCopyNoCache-8   121ms ± 1%   120ms ± 2%  -0.46%  (p=0.000 n=97+88)

Change-Id: Ifcbbb05d773178fad84cb11a9a6768ace69fcf24
Reviewed-on: https://go-review.googlesource.com/94029
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/symtab.go

@@ -853,13 +853,9 @@ func step(p []byte, pc *uintptr, val *int32, first bool) (newp []byte, ok bool)
 	if uvdelta&0x80 != 0 {
 		n, uvdelta = readvarint(p)
 	}
+	*val += int32(-(uvdelta & 1) ^ (uvdelta >> 1))
 	p = p[n:]
-	if uvdelta&1 != 0 {
-		uvdelta = ^(uvdelta >> 1)
-	} else {
-		uvdelta >>= 1
-	}
-	vdelta := int32(uvdelta)
+
 	pcdelta := uint32(p[0])
 	n = 1
 	if pcdelta&0x80 != 0 {
@@ -867,7 +863,6 @@ func step(p []byte, pc *uintptr, val *int32, first bool) (newp []byte, ok bool)
 	}
 	p = p[n:]
 	*pc += uintptr(pcdelta * sys.PCQuantum)
-	*val += vdelta
 	return p, true
 }