cmd/asm: enable MOVSD in the encoding end-to-end test

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>

src/cmd/asm/internal/asm/testdata/amd64enc.s

@@ -2687,18 +2687,18 @@ TEXT asmtest(SB),DUPOK|NOSPLIT,$0
 	MOVQOZX M3, X11                         // f3440fd6db
 	MOVSB                                   // a4
 	MOVSL                                   // a5
-	//TODO: MOVSD (BX), X2                  // f20f1013
-	//TODO: MOVSD (R11), X2                 // f2410f1013
-	//TODO: MOVSD X2, X2                    // f20f10d2 or f20f11d2
-	//TODO: MOVSD X11, X2                   // f2410f10d3 or f2440f11da
-	//TODO: MOVSD (BX), X11                 // f2440f101b
-	//TODO: MOVSD (R11), X11                // f2450f101b
-	//TODO: MOVSD X2, X11                   // f2440f10da or f2410f11d3
-	//TODO: MOVSD X11, X11                  // f2450f10db or f2450f11db
-	//TODO: MOVSD X2, (BX)                  // f20f1113
-	//TODO: MOVSD X11, (BX)                 // f2440f111b
-	//TODO: MOVSD X2, (R11)                 // f2410f1113
-	//TODO: MOVSD X11, (R11)                // f2450f111b
+	MOVSD (BX), X2                          // f20f1013
+	MOVSD (R11), X2                         // f2410f1013
+	MOVSD X2, X2                            // f20f10d2 or f20f11d2
+	MOVSD X11, X2                           // f2410f10d3 or f2440f11da
+	MOVSD (BX), X11                         // f2440f101b
+	MOVSD (R11), X11                        // f2450f101b
+	MOVSD X2, X11                           // f2440f10da or f2410f11d3
+	MOVSD X11, X11                          // f2450f10db or f2450f11db
+	MOVSD X2, (BX)                          // f20f1113
+	MOVSD X11, (BX)                         // f2440f111b
+	MOVSD X2, (R11)                         // f2410f1113
+	MOVSD X11, (R11)                        // f2450f111b
 	MOVSHDUP (BX), X2                       // f30f1613
 	MOVSHDUP (R11), X2                      // f3410f1613
 	MOVSHDUP X2, X2                         // f30f16d2