Floating point test instructions allow special cases (NaN, ±∞ and
a few other useful properties) to be checked directly.

This CL adds the following instructions to the assembler:
 * LTEBR - load and test (float32)
 * LTDBR - load and test (float64)
 * TCEB  - test data class (float32)
 * TCDB  - test data class (float64)

Note that I have only added immediate versions of the 'test data
class' instructions for now as that's the only case I think the
compiler will use.

Change-Id: I3398aab2b3a758bf909bd158042234030c8af582
Reviewed-on: https://go-review.googlesource.com/104457Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>