Add x86 encoding for vconst

cranelift/filetests/filetests/isa/x86/compile-vconst.clif

@@ -0,0 +1,16 @@
+test compile
+set enable_simd=true
+set probestack_enabled=false
+target x86_64 haswell
+
+; use baldrdash calling convention here for simplicity (avoids prologue, epilogue)
+function %test_vconst_i32() -> i32x4 baldrdash_system_v {
+ebb0:
+    v0 = vconst.i32x4 0x1234
+    return v0
+}
+
+; check:   ebb0:
+; nextln:     v0 = vconst.i32x4 0x1234
+; nextln:     return v0
+; nextln: }

cranelift/filetests/filetests/isa/x86/vconst.clif

@@ -0,0 +1,11 @@
+test binemit
+set opt_level=best
+set enable_simd
+target x86_64
+
+function %test_vconst_b8() {
+ebb0:
+[-, %xmm2]  v0 = vconst.b8x16 0x00  ; bin: 0f 10 15 00000008 PCRelRodata4(15)
+[-, %xmm3]  v1 = vconst.b8x16 0x01  ; bin: 0f 10 1d 00000011 PCRelRodata4(31)
+            return
+}