html/template: micro optimization for isJSType

There is an unnecessary lower operation in isJSType.
Simple logic fix can improve tiny performance.

name        old time/op    new time/op    delta
isJSType-8     152ns ± 0%      58ns ± 7%   -61.82%  (p=0.001 n=6+8)

name        old alloc/op   new alloc/op   delta
isJSType-8     32.0B ± 0%      0.0B       -100.00%  (p=0.000 n=8+8)

name        old allocs/op  new allocs/op  delta
isJSType-8      1.00 ± 0%      0.00       -100.00%  (p=0.000 n=8+8)

Change-Id: I281aadf1677d4377920c9649af206381189a27e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/177118


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>