Add x86 implementation of fcvt_from_sint

cranelift/filetests/filetests/isa/x86/simd-conversion-binemit.clif

@@ -9,3 +9,9 @@ block0:
 [-, %xmm2]  v2 = raw_bitcast.i32x4 v1       ; bin:
             return
 }
+
+function %fcvt_32(i32x4) {
+block0(v0: i32x4 [%xmm6]):
+[-, %xmm2]  v1 = fcvt_from_sint.f32x4 v0    ; bin: 40 0f 5b d6
+            return
+}

cranelift/filetests/filetests/isa/x86/simd-conversion-run.clif

@@ -0,0 +1,14 @@
+test run
+set enable_simd
+
+function %fcvt_from_sint() -> b1 {
+block0:
+    v0 = vconst.i32x4 [-1 0 1 123456789]
+    v1 = fcvt_from_sint.f32x4 v0
+
+    v2 = vconst.f32x4 [-0x1.0 0.0 0x1.0 0x75bcd18.0] ; 123456789 rounds to 123456792.0, an error of 3
+    v3 = fcmp eq v1, v2
+    v4 = vall_true v3
+    return v4
+}
+; run