cmd/asm: new x86 instructions

[rsc]

We've had a request on golang-dev for

LDDQU
PSLLDQ
PSRLDQ
PMOVZXBD
PMULLD
PHADDD
PADDD
CVTDQ2PS

[gopherbot]

CL https://golang.org/cl/18312 mentions this issue.

[gopherbot]

CL https://golang.org/cl/18311 mentions this issue.

[rsc]

Also HADDPS, HADDPD.

[gopherbot]

CL https://golang.org/cl/18823 mentions this issue.

[gopherbot]

CL https://golang.org/cl/18850 mentions this issue.

[gopherbot]

CL https://golang.org/cl/18844 mentions this issue.

[gopherbot]

CL https://golang.org/cl/18853 mentions this issue.

[gopherbot]

CL https://golang.org/cl/18900 mentions this issue.