Implement IaddPairwise for the interpreter

Implemented `IaddPairwise` for the Cranelift interpreter, to add pairs
of adjacent values in two SIMD vectors, concatenating them at the end
(preserving both lane size and number of lanes).

Copyright (c) 2021, Arm Limited

cranelift/filetests/filetests/runtests/simd-iaddpairwise.clif

@@ -0,0 +1,25 @@
+test interpret
+test run
+target aarch64
+
+function %iaddp_i8x16(i8x16, i8x16) -> i8x16 {
+block0(v0: i8x16, v1: i8x16):
+    v2 = iadd_pairwise v0, v1
+    return v2
+}
+; run: %iaddp_i8x16([1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16], [17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32]) == [3 7 11 15 19 23 27 31 35 39 43 47 51 55 59 63]
+
+function %iaddp_i16x8(i16x8, i16x8) -> i16x8 {
+block0(v0: i16x8, v1: i16x8):
+    v2 = iadd_pairwise v0, v1
+    return v2
+}
+; run: %iaddp_i16x8([1 2 3 4 5 6 7 8], [100 99 98 97 96 95 94 93]) == [3 7 11 15 199 195 191 187]
+
+function %iaddp_i32x4(i32x4, i32x4) -> i32x4 {
+block0(v0: i32x4, v1: i32x4):
+    v2 = iadd_pairwise v0, v1
+    return v2
+}
+; run: %iaddp_i32x4([1 2 3 4], [5 6 7 8]) == [3 7 11 15]
+; run: %iaddp_i32x4([4294967290 5 4294967290 5], [100 100 100 100]) == [4294967295 4294967295 200 200]