MOVSD is properly handled but its encoding test wasn't enabled. Enable
it.

For reference this was found with a little tool I wrote [1] to explore
which instructions are missing or not tested in the go obj package and
assembler:

"which SSE2 instructions aren't tested? And don't list instructions
which can take MMX operands"

$ x86db-gogen list --extension SSE2 --not-tested --not-mmx
CLFLUSH mem           [m:  np 0f ae /7] WILLAMETTE,SSE2
MOVSD   xmmreg,xmmreg [rm: f2 0f 10 /r] WILLAMETTE,SSE2
MOVSD   xmmreg,xmmreg [mr: f2 0f 11 /r] WILLAMETTE,SSE2
MOVSD   mem64,xmmreg  [mr: f2 0f 11 /r] WILLAMETTE,SSE2
MOVSD   xmmreg,mem64  [rm: f2 0f 10 /r] WILLAMETTE,SSE2

(CLFLUSH was introduced with SSE2, but has its own CPUID bit)

[1] https://github.com/dlespiau/x86db

Change-Id: Ic3af3028cb8d4f02e53fdebb9b30fb311f4ee454
Reviewed-on: https://go-review.googlesource.com/42814Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>