cmd/compile/internal/gc: simplify `x = x <op> y` to `x <op>= y`

Change-Id: I5afba2c10372252be4b65dae7a95461722de904f
Reviewed-on: https://go-review.googlesource.com/135835Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -1419,7 +1419,7 @@ func describeEscape(em uint16) string {
 		}
 		s += "contentToHeap"
 	}
-	for em >>= EscReturnBits; em != 0; em = em >> bitsPerOutputInTag {
+	for em >>= EscReturnBits; em != 0; em >>= bitsPerOutputInTag {
 		// See encoding description above
 		if s != "" {
 			s += " "
@@ -1469,7 +1469,7 @@ func (e *EscState) escassignfromtag(note string, dsts Nodes, src, call *Node) ui
 
 	em0 := em
 	dstsi := 0
-	for em >>= EscReturnBits; em != 0 && dstsi < dsts.Len(); em = em >> bitsPerOutputInTag {
+	for em >>= EscReturnBits; em != 0 && dstsi < dsts.Len(); em >>= bitsPerOutputInTag {
 		// Prefer the lowest-level path to the reference (for escape purposes).
 		// Two-bit encoding (for example. 1, 3, and 4 bits are other options)
 		//  01 = 0-level

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

@@ -1312,7 +1312,7 @@ opswitch:
 						b = conv(b, convType)
 						b = nod(OLSH, b, nodintconst(int64(8*offset)))
 						ncsubstr = nod(OOR, ncsubstr, b)
-						csubstr = csubstr | int64(s[i+offset])<<uint8(8*offset)
+						csubstr |= int64(s[i+offset]) << uint8(8*offset)
 					}
 					csubstrPart := nodintconst(csubstr)
 					// Compare "step" bytes as once