runtime: Simplify slice growing/appending code (#4287)

* reflect: rawFieldByNameFunc: copy index slice to avoid later overwrites

* runtime: Simplify slice growing/appending code

Refactor the slice appending function to rely on the slice growing
function, and remove branches/loops to use a branchfree variant.

Signed-off-by: L. Pereira <l.pereira@fastly.com>

* runtime: Remove one branch in sliceAppend()

Both branches were equivalent, so guard the overall logic in
sliceAppend() with the more general condition.

Signed-off-by: L. Pereira <l.pereira@fastly.com>

* runtime: Simplify slice growing calculation

Use `bits.Len()` rather than `32 - bits.LeadingZeros32()`.  They're
equivalent, but the Len version is a bit easier to read.

Signed-off-by: L. Pereira <l.pereira@fastly.com>

* reflect: Always call sliceGrow() in extendSlice()

sliceGrow() will return the old slice if its capacity is large enough.

Signed-off-by: L. Pereira <l.pereira@fastly.com>

---------

Signed-off-by: L. Pereira <l.pereira@fastly.com>
Co-authored-by: Damian Gryski <damian@gryski.com>

src/reflect/value.go

@@ -1712,18 +1712,7 @@ func extendSlice(v Value, n int) sliceHeader {
 		old = *(*sliceHeader)(v.value)
 	}
 
-	var nbuf unsafe.Pointer
-	var nlen, ncap uintptr
-
-	if old.len+uintptr(n) > old.cap {
-		// we need to grow the slice
-		nbuf, nlen, ncap = sliceGrow(old.data, old.len, old.cap, old.cap+uintptr(n), v.typecode.elem().Size())
-	} else {
-		// we can reuse the slice we have
-		nbuf = old.data
-		nlen = old.len
-		ncap = old.cap
-	}
+	nbuf, nlen, ncap := sliceGrow(old.data, old.len, old.cap, old.len+uintptr(n), v.typecode.elem().Size())
 
 	return sliceHeader{
 		data: nbuf,