Add scalar_to_vector instruction

Moves scalar values in a GPR register to an FPR register

cranelift/filetests/filetests/isa/x86/scalar_to_vector.clif

@@ -0,0 +1,32 @@
+test binemit
+set opt_level=best
+set enable_simd
+target x86_64 has_sse2=true
+
+function %test_scalar_to_vector_b8() {
+ebb0:
+[-, %rax]   v0 = bconst.b8 true
+[-, %xmm0]  v1 = scalar_to_vector.b8x16 v0    ; bin: 66 0f 6e c0
+            return
+}
+
+function %test_scalar_to_vector_i16() {
+ebb0:
+[-, %rbx]   v0 = iconst.i16 42
+[-, %xmm2]  v1 = scalar_to_vector.i16x8 v0    ; bin: 66 0f 6e d3
+            return
+}
+
+function %test_scalar_to_vector_f32() {
+ebb0:
+[-, %rcx]   v0 = f32const 0x0.42
+[-, %xmm3]  v1 = scalar_to_vector.f32x4 v0    ; bin: 66 0f 6e d9
+            return
+}
+
+function %test_scalar_to_vector_i64() {
+ebb0:
+[-, %rdx]   v0 = iconst.i64 42
+[-, %xmm7]  v1 = scalar_to_vector.i64x2 v0    ; bin: 66 0f 6e fa
+            return
+}