Add x86-specific shuffle instructions

This includes both PSHUFD and PSHUFB; these are necessary to legalize future SIMD instructions.

cranelift/filetests/filetests/isa/x86/pshufb.clif

@@ -0,0 +1,13 @@
+test binemit
+set enable_simd
+target x86_64 has_sse2=true has_ssse3=true
+
+function %test_pshufb() {
+ebb0:
+[-, %rax]   v0 = iconst.i8 42
+[-, %xmm0]  v1 = scalar_to_vector.i8x16 v0   ; bin: 66 40 0f 6e c0
+[-, %rbx]   v2 = iconst.i8 43
+[-, %xmm4]  v3 = scalar_to_vector.i8x16 v2   ; bin: 66 40 0f 6e e3
+[-, %xmm0]  v4 = x86_pshufb v1, v3               ; bin: 66 0f 38 00 c4
+            return
+}

cranelift/filetests/filetests/isa/x86/pshufd.clif

@@ -0,0 +1,11 @@
+test binemit
+set enable_simd
+target x86_64 has_sse2=true
+
+function %test_pshuf() {
+ebb0:
+[-, %rax]   v0 = iconst.i32 42
+[-, %xmm0]  v1 = scalar_to_vector.i32x4 v0  ; bin: 66 40 0f 6e c0
+[-, %xmm0]  v2 = x86_pshufd v1, 0                ; bin: 66 0f 70 c0 00
+            return
+}