cmd/compile: inline x, ok := y.(T) where T is a scalar

When T is a scalar, there are no runtime calls
required, which makes this a clear win.

encoding/binary:
WriteInts-8                958ns ± 3%     864ns ± 2%   -9.80%  (p=0.000 n=15+15)

This also considerably shrinks a core fmt
routine:

Before: "".(*pp).printArg t=1 size=3952 args=0x20 locals=0xf0
After:  "".(*pp).printArg t=1 size=2624 args=0x20 locals=0x98

Unfortunately, I find it very hard to get stable
numbers out of the fmt benchmarks due to thermal scaling.

Change-Id: I1278006b030253bf8e48dc7631d18985cdaa143d
Reviewed-on: https://go-review.googlesource.com/26659


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>