internal/load: support additional MOVQ forms

The Go assembler merges MOVD/MOVQ instruction forms. The logic in the
avo instruction loader was discarding the MOVD forms. This diff should
merge them correctly.

Updates #50

tests/fixedbugs/issue50/asm.go

@@ -0,0 +1,21 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+)
+
+func main() {
+	TEXT("Issue50", NOSPLIT, "func(x uint32) uint32")
+	Doc(
+		"Issue50 reported that MOVD/MOVQ was missing the r32, xmm form.",
+		"This function deliberately exercises this instruction form.",
+	)
+	x := Load(Param("x"), GP32())
+	xmm := XMM()
+	MOVQ(x, xmm)
+	Store(xmm, ReturnIndex(0))
+	RET()
+	Generate()
+}