Explcitly block fused multiply-add pattern matching when a cast is used
after the multiplication, for example:

    - (a * b) + c        // can emit fused multiply-add
    - float64(a * b) + c // cannot emit fused multiply-add

float{32,64} and complex{64,128} casts of matching types are now kept
as OCONV operations rather than being replaced with OCONVNOP operations
because they now imply a rounding operation (and therefore aren't a
no-op anymore).

Operations (for example, multiplication) on complex types may utilize
fused multiply-add and -subtract instructions internally. There is no
way to disable this behavior at the moment.

Improves the performance of the floating point implementation of
poly1305:

name         old speed     new speed     delta
64           246MB/s ± 0%  275MB/s ± 0%  +11.48%   (p=0.000 n=10+8)
1K           312MB/s ± 0%  357MB/s ± 0%  +14.41%  (p=0.000 n=10+10)
64Unaligned  246MB/s ± 0%  274MB/s ± 0%  +11.43%  (p=0.000 n=10+10)
1KUnaligned  312MB/s ± 0%  357MB/s ± 0%  +14.39%   (p=0.000 n=10+8)

Updates #17895.

Change-Id: Ia771d275bb9150d1a598f8cc773444663de5ce16
Reviewed-on: https://go-review.googlesource.com/36963
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>