Add x86 encoding for SIMD imul

Only i16x8 and i32x4 are encoded in this commit mainly because i8x16 and i64x2 do not have simple encodings in x86. i64x2 is not required by the SIMD spec and there is discussion (https://github.com/WebAssembly/simd/pull/98#issuecomment-530092217) about removing i8x16.

cranelift/filetests/filetests/isa/x86/simd-arithmetic.clif

@@ -120,3 +120,47 @@ ebb0:
 
     return ; bin: c3
 }
+
+function %imul_i32x4() -> b1 {
+ebb0:
+[-, %xmm0]    v0 = vconst.i32x4 [-1 0 1 -2147483647] ; e.g. -2147483647 == 0x80_00_00_01
+[-, %xmm1]    v1 = vconst.i32x4 [2 2 2 2]
+[-, %xmm0]    v2 = imul v0, v1 ; bin: 66 0f 38 40 c1
+
+    v3 = extractlane v2, 0
+    v4 = icmp_imm eq v3, -2
+
+    v5 = extractlane v2, 1
+    v6 = icmp_imm eq v5, 0
+
+    v7 = extractlane v2, 3
+    v8 = icmp_imm eq v7, 2 ; 0x80_00_00_01 * 2 == 0x1_00_00_00_02 (and the 1 is dropped)
+
+    v9 = band v4, v6
+    v10 = band v8, v9
+    return v10
+}
+; run
+
+function %imul_i16x8() -> b1 {
+ebb0:
+[-, %xmm1]    v0 = vconst.i16x8 [-1 0 1 32767 0 0 0 0] ; e.g. 32767 == 0x7f_ff
+[-, %xmm2]    v1 = vconst.i16x8 [2 2 2 2 0 0 0 0]
+[-, %xmm1]    v2 = imul v0, v1 ; bin: 66 0f d5 ca
+
+    v3 = extractlane v2, 0
+    v4 = icmp_imm eq v3, 0xfffe ; TODO -2 will not work here and below because v3 is being
+    ; uextend-ed, not sextend-ed
+
+    v5 = extractlane v2, 1
+    v6 = icmp_imm eq v5, 0
+
+    v7 = extractlane v2, 3
+    v8 = icmp_imm eq v7, 0xfffe ; 0x7f_ff * 2 == 0xff_fe
+
+    v9 = band v4, v6
+    v10 = band v8, v9
+
+    return v4
+}
+; run