runtime: simplify hashmap tooManyOverflowBuckets

This generates better code.

Masking B in the return statement should be unnecessary,
but the compiler is understandably not yet clever enough to see that.

Someday, it'd also be nice for the compiler to generate
a CMOV for the saturation if statement.

Change-Id: Ie1c157b21f5212610da1f3c7823a93816b3b61b9
Reviewed-on: https://go-review.googlesource.com/54656
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>

src/runtime/hashmap.go

@@ -995,10 +995,11 @@ func tooManyOverflowBuckets(noverflow uint16, B uint8) bool {
 	// If the threshold is too high, maps that grow and shrink can hold on to lots of unused memory.
 	// "too many" means (approximately) as many overflow buckets as regular buckets.
 	// See incrnoverflow for more details.
-	if B < 16 {
-		return noverflow >= uint16(1)<<B
+	if B > 15 {
+		B = 15
 	}
-	return noverflow >= 1<<15
+	// The compiler doesn't see here that B < 16; mask B to generate shorter shift code.
+	return noverflow >= uint16(1)<<(B&15)
 }
 
 // growing reports whether h is growing. The growth may be to the same size or bigger.